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Synthetic Communications: AnInternational Journal for RapidCommunication of SyntheticOrganic ChemistryPublication details, including instructions forauthors and subscription information:http://www.tandfonline.com/loi/lsyc20

Further Studies on QuinoneDiels-Alder Reactionswith 1,3,3-Trimethyl-2-vinylcyclohexenes:Regioselective Synthesisof 12-Methyl-podocarpaneDiterpenes and Isolation of aHetero Diels-Alder Productfrom 1,4-BenzoquinoneThomas A. Engler a , UmaShanker Sampath a , DavidVander Velde a & Fusao Takusagawa aa Department of Chemistry, University of Kansas,Lawrence, KS, 66045, USAPublished online: 23 Sep 2006.

To cite this article: Thomas A. Engler , UmaShanker Sampath , David Vander Velde& Fusao Takusagawa (1992): Further Studies on Quinone Diels-Alder Reactionswith 1,3,3-Trimethyl-2-vinylcyclohexenes: Regioselective Synthesis of 12-Methyl-podocarpane Diterpenes and Isolation of a Hetero Diels-Alder Product from 1,4-Benzoquinone, Synthetic Communications: An International Journal for RapidCommunication of Synthetic Organic Chemistry, 22:14, 2031-2042

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SYNTHETIC COMMUNICATIONS, 22(14), 2031-2042 (1992)

FURTHER STUDIES ON QUINONE DIELS-ALDER REACIlONS WITH 1,3,3-TRIMEI'EYL-2-VINYLCYCJBFIEXENES REGIOSELECTIVE SYNTHESIS OF 12-METHYL-PODOCARPANE DITERPENES AND ISOLATION OF A HETERO DIELS-ALDER PRODUCT FROM 1,4-

BENZOQUMONE

Thomas A. Engler,* UmaShanker Sampath, David Vander Velde and Fusao Takusagawa

Department of Chemistry, University of Kansas, Lawrence, KS 66045 USA

Abstract Diels-Alder reactions of 2-[(E)-B-alkoxyvinylJ-l,3,3-trimethyl- cyclohexenes with 2-methoxy-3-methyl-l,4-benzoquinone regioselectively pro- duce adducts possessing the 12-methyl-podocarpane ring skeleton. In addition, in the reaction with 1,4-benzoquinone an unusual hetero Diels-Alder product involving the C=O group of the quinone is found in low yield.

Recently, we reported a new synthetic approach to podocarpane and

abietane diterpenes via regioselective Lewis acidhigh pressure promoted

Diek-Alder reactions of 1,3,3-trimethyl-2-vinylcyclohexene, la, with 2-methoxy-

1,Qbenzoquinones 2 a b to give 3ab, respectively.' A number of naturally

occurring trimethyl-perhydrophenanthrene diterpenes also possess oxygen

substitutents at C-7.2 In order to extend the synthetic utility of our approach,

* To whom correspondence should be addressed.

2031

Copyright 0 1992 by Marcel Dekker, Inc.

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2032 ENGLER ET AL.

l a , X-H 2a. R=H 3a. X-R-H &, X-R-H

lc, X-OCH3 2c, RICH, a, XIOAC, RICH, 4c, XIOAe, R - C k 1 b, X-OAC 2b, R-CH(CH& 3b, X-H, R-CH(CH& 4b, X-H, R=CH(CH&

3d. XIOCH3, RlCH3 4d, X-OCH3, R e

we explored the Diels-Alder reactions of the oxygen substituted dienes lb/c.

Our initial studies showed that although reactions of l a with 2a were highly

stereo(end0)- and regioselective, those of l b or lc with 2a proceeded with

high endo stereoselectivity but modest regioselectivity (3:4=61 to 1:2).'

Herein, we report the results of reactions of lb/c with 2-methoxy-3-methyl-l,4-

benzoquinone, 2c: in which regioisomer 4 can be obtained in good yield with

> 1O:l selectivity. We also report the isolation and structural characterization

of an unusual hetero Diels-Alder adduct from reaction of 1,Cbenzoquinone

with lc.

Reactions of Ib4 or lc with 2c were examined under combined Lewis

acidhigh pressure conditions (Table I).' Previous studies have shown that

Diels-Alder reactions under these conditions proceed with greater selectivity

than observed with either promoting technique alone. The most selective

reaction was that between l c and 2c in which a <1:10 ratio of 3d:4d was

found in 74% yield. These reactions hold considerable promise as a strategic

method to stereoselectively prepare 12-alkyl-podocarpane diterpenes.

The stereochemistry of the adducts 3 and 4 as that resulting from endo

addition of the diene and the quinone is assigned by 'H NMR as previously

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QUINONE DIELS-ALDER REACTIONS 2033

Table I. Diels-Alder Reactions of 2-Methoxy-3-methyl-l,4-benzoquinone (2c) with Dienes l b and lc.

Entry Diene Pressure (kbar) Lewis Acid (equiv)a Time % Yields 3 c k

1 l b 14 none 6 24 51

2 l b 14 ZnBr, (0.19) 6 14 50

3 lb 12-14 Yb(fOdh(O.05) 6 24 53

3d 4d 4 IC 12-14 ZnBr, (0.20) 6 <6 >67

a) With respect to quinone.

discussed.' All carbon and proton resonances are assigned from DEPT,

HETCOR and 'H-'H decoupling experiments. Thus, the axial C-1 hydrogen

is evident as a downfield ddd (Jgem = -13, JaX-= = -13, J,+ = -4.5 Hz)

at -2.5 ppm and a cis B/C ring fusion is indicated by a H-8/H-9 coupling

constant -5.5Hz. The stereochemistry of the acetoxy group in 3c/& and the

methoxy group in 4d is assigned as trans relative to H-8 based on a -6.7 Hz

coupling constant between H-7 and H-8 which is consistent with a cis H-7/H-8

relationship. If H-7 and H-8 were trans, a larger trans diaxial coupling

constant would be expected.

The positions of the substituents on ring C of 3c/d and 4d are assigned

by heteronuclear ( 'H-13C) multiple-bond correlation (HMBC) NMR

experiments.' HMBC spectra reveal two-bond and three-bond C-H

connectivities. Proton and carbon nuclei directly attached or separated by

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2034 ENGLER ET AL.

Table 11. Summary of Heteronuclear Multiple-Bond Correlation Studies on 4c and 4d.

16

21 14 19

4c

correlated protons; Carbon# (DDRl) DrOtOn# fDDm)

l(33.6) 2(18.0)

4(35.6)* S(156.3) 6( 115.2) 7(67.1) 8(47.4) g(56.9) lO(35.4) ll(197.9) 12(137.7) 13(157.9) 14( 193.6) lS(59.8) 16(10.1) 17(29.6) 18(30.7)' 19( 3 1.2) 20( 169.4) 21(20.9)

3(39.3)

17(1.23) la,3aJB( 1.47) 18(1.13),19(1.13) 18,19,6(5.47) 7,9(2.88),17,18,19 7(5.52) 9 6,9 la(2.52),7,8(3.67),17 18,19,6(5.47) 8,9,16( 1.94) 16 8,15(3.92),16 8

10,9 30(1.67) 3a 7,21( 1.86)

4d

carrelated protons; carbon# IDDm) proton# born)

l(33.6) 2(18.1) 3(39.3) 18(1.13),19(1.17) 4(35.3) 18,19,6(5.63) S(154.5) 9,17(1.18),18,19

7(74.4) 8,9,20(3.20) 8(48.8) 6,9(2.78) g(56.9) 1B( -1.15),17,8,(3.59) lO(35.6) 1B,la(2.56),9,17 11( 198.3) 8,9,16(1.91) 12(138.2) 16 13(158.7) 8,15(3.88),16 14( 195.2) 8 lS(58.7) 16(10.2) 17(29.7) la,lB,9 18(31.0) 19(31.3) 20(57.6)

*carbon resonances too close - no correlations observed

6(115.4) 7(3.95)

more than 3 bonds are not correlated (of course, direct C-H connectivities are

identified in the HETCOR spectra). The data are summarized in Table 11.

For compounds Wd, the most indicative couplings are the following: 1) the

protons on the C-15 methoxy group are coupled only to C-13 (157.9 and 158.7

ppm, respectively); 2) the C-16 methyl protons are coupled to C-ll(197.9 and

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QUINONE DIELS-ALDER REACTIONS 2035

198.3 ppm, respectively), C-12 (137.7 and 138.2 ppm, respectively) and C-13,

and 3) H-8 is coupled to C-13. Based on an examination of molecular

models, the dihedral angle between H-9 and C-12 is -85" which explains the

lack of coupling between them. All other data are consistent with structures

4c and 4d. Adduct 3c is the only other likely isomer which fits the data given

in the previous paragraph.

While further optimizing the reaction of diene l c with 1,4

benzoquinone, and thus its potential utility in the synthesis of natural products,

we found that the pressure promoted reaction (12 kbar, 96 hr) produces, in

addition to the previously reported Diels-Alder products 5 and 6 in 48% and

3% yields, respectively, the unusual adduct 7 in 12% yield. Dienone 7 is 0

1c 5 6 7

apparently formed via a formal hetero Diels-Alder reaction of the quinone-

carbonyl group with the diene. The structure of 7 was determined by single

crystal X-ray analysis (Figure). Hetero Diels-Alder reactions involving the

carbonyl group of a 1,Cbenzoquinone are rare6 We are aware of only three

reported examples and they involve reactions of electron deficient dichloro-

quinones or a quinoline-quinone with a vinyl ketene acetal' and a 3,3

dioxygenated-2-azadiene,' respectively. Reactions of other 1,Cbenzoquinones

with oxygenated dienes and alkenes have been utilized extensively in synthesis;

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2036 ENGLER El' AL.

A B

Fig. A - ORTEP diagram of 7: B - PLUTO diqram Wim hydrogen atoms omitted for darity.

however, to our knowledge, products of hetero Diels-Alder reactions have not

been identifiedg*" In addition, reaction of the quinone with lc at the steri-

cally hindered C-1 position is noteworthy. This result may imply that hetero

Diels-Alder reactions of dienes and 1,4-benzoquinones may be more common

than generally thought.

EXPERIMENTAL.^^

Preparation of 4d. Pressurization of a dichloromethane solution (2mL) of a

1O:l mixture of enol ether l c and its Z-isomer (42 mg, 0.21 mmol of lc),

quinone 2c (50 mg, 0.33 mmol) and ZnBr, (14.5 mg, 0.064 mmol) to 12-14

kbar for 6 d gave, after filtration of the mixture through Celite, concentration

of the filtrate and chromatography of the residue on a 2mm SO,

chromatotron plate at a 1 mm flow rate with 1: lO EtOAchexanes as eluent,

a >10:1 mixture of adducts 4d and 3d (52 mg, 74%) as a clear oil. Physical

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QUINONE DIELS-ALDER REACTIONS 2037

and spectral data for 4d: Rf 0.29 (20% EtOAchexanes). 'H NMR (CDCl,,

500 MHz) 5.63 (d, 1H,J = 3.7 Hz, H-6), 3.95 (dd, 1H,J = 6.8,3.7 Hz, H-7),

3.88 (s, 3H, H-15), 3.59 (dd, lH,J = 6.8,5.5 Hz, H-8), 3.20 (s, 3H, H-20), 2.78

(d, lH, J = 5.5 Hz, H-9), 2.56 (ddd, 1H,J = 12.8, 12.8, 4.6 Hz, H-l,), 1.91

(s, 3H, H-16), 1.81 (ddddd, lH,J = 3.5, 3.5, 13.1, 13.1, 13.1 Hz, H-2,), 1.70

(ddd, lH, J = 3.2, 12.9, 12.9 Hz, H-3,), 1.59 (m, lH, J = 13.1 Hz, H-2q),

1.46 (m, lH, H-3q), 1.2-1.1 (m, lH, H-lq), 1.18 (s, 3H, H-17), 1.17 (s, 3H,

H-18), 1.13 (s, 3H, H-19). 13C NMR (CDCl,, 75 MHz) 198.3, 195.2, 158.7,

154.5, 138.3, 115.4, 74.3,59.7, 57.6,56.9, 48.8, 39.3, 35.6, 35.3, 33.6, 31.3, 31.0,

29.7, 18.0, 10.2. IR (CDCI,) 2936, 2869, 1681, 1614, 1441, 1316, 1224, 1200,

1131, 1072 cm-'. EIMS m/z (relative intensity) 332 (M+, 2), 304(4), 289(6),

261( 17), 244(4), 230(5), 221(5), 215(4), 201(4), 187(5), 179(100), 165(31),

147(89), 129(99), 121(28), 105(48), 91(51), 85(54), 83(79), 67(30), 55(65),

43(46). HRMS m/z-332.1996 (Calcd for C&H,O4-332.1988). Compound

3d was detected as a minor isomer (c 10%) in the 'H-NMR spectrum of 4d.

Preparation of 3c/4c. A 2.7:l mixture of enol acetate l b and its Z-isomer

(36.9 mg of lb, 0.177 mmol), quinone 2c (50.1 mg, 0.33 mmol) and Yb(fod),

(17.5 mg, 0.0165 mmol) in dichloromethane (2.0 mL) was pressurized to 12-14

kbar for 6 days. Concentration and flash chromatography of the residue on

silica gel with 1 : l O EtOAchexanes as eluent gave enedione 4c (33.8 mg, 53%)

as a white solid and 3c (15.2 mg, 24%) as an oil. Physical and spectral data

for 4c: Rf 0.16 (20% EtOAchexanes). 'H NMR (CDCI,, 300 MHz) 5.52 (dd,

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2038 ENGLER ET AL.

lH, J = 6.6, 3.9 Hz, H-7), 5.47 (d, lH, J = 3.9 Hz, H-6), 3.92 (s, 3H, H-15),

3.67 (dd, lH, J = 6.6, 5.4 Hz, H-8), 2.88 (d, lH, J = 5.4 Hz, H-9), 2.52 (ddd,

lH, J = 13.0, 13.0,4.4 Hz, H-l& 1.94 (s, 3H, H-16), 1.86 (s, 3H, H-21), 1.82

(rn, lH, H-2), 1.67 (ddd, 1H,J = 3.3, 13.0, 13.0 Hz, H-3,), 1.59 (m, 1H,J =

13.3 Hz, H-2), 1.47 (m, IH, J = 13.0 Hz, H-3q), 1.23 (s, 3H, H-17), 1.18 (m,

lH, J = 13.0 Hz, H-lq), 1.13 (s, 6H, H-18 and H-19). 13C NMR (CDC13, 75

MHz) 197.9, 193.6, 169.4, 157.9, 156.4, 137.8, 115.2,67.1,59.8,56.9,47.4, 39.3,

35.6,35.4,33.6, 31.2,30.8,29.6,20.9, 17.9, 10.1. IR (CDC13) 2937,2870, 1732,

1683, 1612, 1315, 1235, 1208, 1132, 1071, 1021 cm-l. EIMS m h -(relative

intensity) 360 (M+, 5) , 318(16), 285(10), 244(11), 207(12), 165(23), 154(24),

107(21), 105(17), 91(21), 83(45), 55(28), 43(100). HRMS m/z- 360.1939

(Calcd for C2,HBO5-360.1937). Physical and spectral data for 3c: Rf 0.14

(20% EtOAchexanes). 'H NMR (CDCI,, 300 MHz), 5.57 (dd, lH, J = 6.6,

4.0 Hz, H-7), 5.49 (d, 1H,J = 4.0 Hz, H-6), 3.97 (s, 3H, H-15), 3.74 (dd, lH,

J = 6.8, 5.4 Hz, H-8), 2.98 (d, 1H,J = 5.4 Hz, H-9), 2.47 (ddd, lH, J = 12.9,

12.9, 4.5 Hz, H-lm), 1.86 (s, 3H, H-16), 1.84 (s, 3H, H-21), 1.9-1.8 (m, lH),

1.72 (ddd, 1H,J = 3.4, 13.1, 13.1 Hz), 1.64 (m, lH,J = 13.6 Hz), 1.52 (m, lH,

J = 12.8 Hz), 1.25 (S 3H, H-17), 1.26-1.23 (m, lH), 1.17 (s, 3H, H-18), 1.16 (s,

3H, H-19). 13C NMR (CDC13, 75 MHz) 197.2, 194.2, 169.4, 163.2, 155.8;

128.8, 115.8,66.9, 59.3,57.4,49.1,39.4, 35.41,35.37,33.7,31.3,30.9, 29.5,20.8,

17.9, 8.5. IR (CDCl,) 2938, 2869, 1733, 1707, 1663, 1607, 1285, 1236, 1208,

1122, 1029 ern-'. EIMS mh (relative intensity) 360 (M+, l), 317(2), 284(12),

272( 13), 257( lo), 230(12), 164(10), 149(28), 128(10), 115(15), 105(16), 91( 17),

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QUINONE DIELS-ALDER REACTIONS 2039

83(36), 55(32), 43(100).

360.1937).

HRMS mlz - 360.1946 (Calcd for q1HB05 -

Preparation of 7. A solution of 2-[(E)-(l3-methoxyvinyl)]-l,3,3-

trimethylcyclohexene (103 mg, 0.57 mmol) and l,+benzoquinone (48 mg, 0.44

mmol) in dichloromethane (2 LUL) was pressurized to 12 kbar for 96 hours.

The reaction mixture was concentrated and the residue chromotographed on

silica gel with lo%, 14% and then 25% ethyl acetatehexanes as eluent to give

enedione 5 (62 mg, 48%), mp 114-115°C (EtOAchexanes), spiropyran 7 (15

mg, 12%), mp 8687°C (EtOAchexanes) and quinone 6 (4 mg, 3%).

Compounds 5 and 6 were characterized as previously reported.' Spectral data

for 7: 'H NMR (CDCI,, 500 MHz) 6 1.17 (s, 3H), 1.21 (s, 3H), 1.38 (s, 3H),

1.05-1.75 (m, 6H), 3.35 (s, 3H, OCH,), 5.03 (d,J = ~ H z , lH, CH-OCH,), 5.69

(d, J = 2 Hz, lH, C=CH-), 6.21 (dd, J = 10.6, 2.0 Hz, lH, CH=CH-C=O),

6.30 (dd, J = 10.5,2.0 Hz, lH, -CH=CH-C=O), 6.93 (dd, J = 10.5, ~ H z , lH, - CH=CH-C=O), 7.33 (dd, J = 10.6, 3 Hz, lH, -CH=CB-C=O). I3C NMR

(125 MHz, CDCI, reported in ppm from residual CHCI, as internal standard)

6 17.28, 23.81, 31.75, 31.79, 31.84, 34.96, 39.23, 41.02, 55.01, 73.79, 95.54,

117.83, 128.52, 130.58, 146.88, 149.51, 151.46, 185.79. IR (CCl,) 2960(br),

2890(m), 2840(w), 1670(s), 1635(m), 1470(m), 1390(m), 1375(m), 1365(m),

1190(m), 1135(m), 1070(s), 985(m), 950(m), 855(m) cm". EIMS (% intensity)

mh 288 (M+, l), 257 (7), 180 (loo), 165 (59), 133 (28), 124 (15), 119 (lo),

109 (15), 105 (18), 97 (ll), 91 (29), 77 (24), 71 (13), 65 (14), 41 (76). HRMS

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2040 ENGLER ET AL.

mlz - 288.1714 (calcd for C18H2403-288.1725). UV (MeOH, nm) A ( € ) 25

(5600), 224 (ll,lW), 205 (9,200). Rf0.38 (20% EtOAchexanes). Anal. Calcd

for C18H,0,: C, 74.97; H, 8.39. Found: C, 74.95; H, 8.30. Crystal data for

7. Colorless crystal (0.7 x 0.7 x 0.8 mm) obtained by recrystallization from

EtOAchexanes. Empirical Formula -C18H2403. Formula Weight - 288.39.

Crystal System - Monoclinic. Cell Constants: a = 7.4947 (9) b = 10.954

(1) A; c = 9.8199 (8) A; B = 98.990 (7) "; V = 796.3 (1) A3; Space Group -

P2, (#4). Z value - 2. Dale -1.203 g/cm3. Fooo - 312. &,,&) - 0.75 cm-'.

Diffractometer - Rigaku AFCSR. Radiation - MoKa(A = 0.71069 f$ Graphite

- monochromated). Temperature - 23°C. 20,, - 45.0". No. Observations

(bO.01 a(1)) - 1099 (total measured - 1208). No. Variables - 285. Residual -

R; R,: 0.041; 0.048. Goodness of Fit Indicator - 1.396. Maximum Shift of

Positional Paramters in Final Cycle - 0.53. Largest Peak in Final Diff. Fourier

Map - 0.16 e/A3. Structure solved by direct methods. The author has

deposited atomic coordinates for this structure with the Cambridge

Crystallographic Data Centre. The coordinates can be obtained on request

from The Director, Cambridge Crystallographic Data Centre, University

Chemical Laboratory, Lensfield Road, Cambridge CB2 lEW, U.K.

Acknowledgment. This research was supported by the University of Kansas

General Research Fund and the National Institutes of Health.

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QUINONE DIELS-ALDER REACTIONS

References and Notes.

2043

1.

2.

3.

4.

5.

6.

7.

8.

Engler, T.A.; Sampath, U.; Naganathan, S.; Vander Velde, D.;

Takusagawa, F. and Yohannes, D. J. Org. Chem. 1989,

Dev, S.; Renuka, M. "CRC Handbook of Terpenoids-Diterpenoids,

Vol. 111"; CRC Press: Boca Raton F'L, 1986.

a) Ishii, H.; Ohtake, R.; Okida, H.; Mitsui, H. and Ikeda, N. J. Pharm.

SOC. Jpn. 1970, s, 1283. b) Kirkemo, C.L. and White, J.D. J. Org.

Chem. 1985,2, 1318.

Diene lb was prepared as the major fraction of a -2-3:l mixture of lb

and its 2-isomer. The mixture was used as is and recovery and 'H

NMR analysis of starting material indicated that the minor isomer did

not react.l

Bax, A. and Summers, M.F. J. Am. Chern. SOC. 1986,108. 2093.

Photochemical reactions of 1,Cbenzoquinone with dienes are known to

produce dihydrospiropyrans (4+2 cycioadducts involving the C=O

group), presumably via a stepwise process; see a) Barltrop, J.A. and

Hesp, B. J. Chem. SOC., 1965, 5182. b) Bruce, J.M. Quart. Rev.,

Chem. SOC., 1967,2l, 405. c) Wilson, R.M.; Gardner, E.J.; Elder, R.C.;

Squire, R.H. and Florian, L.R. J. Am. Chem. SOC., 1974, & 2955.

a) Guay, V. and Brassard, P. Tetrahedron 1984,

Simoneau, B. and Brassard, P. Tetrahedron, 1986, a 3767. Echavarren, A.M. and Stille, J.K. J. Am. Chem. SOC., 1988,110,4051.

5712.

5039. See also b)

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ENGLER ET A L . 2042

9.

10.

Boisvert, L. and Brassard, P. J. Ore. Chem., 1988, a 4052.

For an extensive review, see Finley, K.T. In 'The Chemistry of the

Quinonoid Compounds, Vol. 2, Part 1" Patai, S. and Rappoport, Z.,

Eds.; WiIey-Interscience, New York, 1988 p. 537.

For general experimental procedures, see reference 1. 11.

(Received in USA 11 March, 1992.)

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