Generation and cycloadditions of hydroxy(and trimethylsiloxy)phenylethynylketene

Tetrahedron Letters,Vol.29,No.23,pp2765-2768,1988 Printed in Great Britain

0040-4039/88 $3.00 + .OO Pergamon Press plc

GENERATION AND CYCLOADDITIONS OF HYDROXY(AND TRIMETHYLSILOXY)PHENYLETHYNYLKETENE Mario Fernandez, Daniel J. Pollart and Harold W. Moore Department of Chemistry, University of California Irvine, California 92717

Abstract:

An unusual rearrangement of 5 , 6 - d i h y d r o x y - l , 3 - d i m e t h o x y - 5 , 6 - d i p h e n y l e t h y n y l - l , 3 -

cyclohexadiene ( 2 a ) was

(1)

to

7-hydroxy-l ,4-dimethoxy-2-phenylethynylbicyclo[2.2.2

when

1

bicyclooctadienones

2a,b

were observed to undergo retro Diels-Alder fragmentation

give

hydroxy(or

alkynylketenes

was

treated

with ZnCl2

in

refluxing

trimethylsiloxy)-phenylethynylketenes

other

than

alkynylcyanoketenes

to

appear

cycloadds to a variety of ketenophiles, but most interestingly, diphenylacetylene functionalized

to

Cyclohexadiene excess mL)

give

the

benzoquinone 1

is

cyclopentenedione

solution

of

1

those

available

(1.34

Furthermore,

from

unambiguous

from

The

at 138 °C to

the only e x a m p l e s

literature. 1

it undergoes

of

The ketene 3 b

unusual

3,6-dimethoxy-l,2-benzoquinone

reactions with

to yield

the

highly

mmol)

and

ZnC12

upon

treatment

with

Surprisingly, when a carbon tetrachloride (25 (0.36

mmol)

was

refluxed

for

2

hr

2a was realized in 60% yield as a white crystaline solid, rap, 141-142 ° C . NOE

and

proof

two-dimensional

analysis

proton-carbon

of its spectral correlation

properties,

NMR

studies.

of the structure was obtained by single crystal X-ray analysis. 3

Details of the rearrangement of 1 to 2a are yet would be 4.

the

7 and with ethoxypropyne

The structure of 2a can be assigned on the basis of a detailed particularly

3a,b, in

tetrachloride.

8.

1-1ithio-2-phenylethyne in THF at -78 ° C . 2

bicyclo[2.2.2]octadienone

carbon

]octadiene-8-one

observed

to be determined,

but a reasonable

intermediate

This is envisaged to arise via protonation of one of the alkynyl groups followed by

attack of the resulting vinyl cation or its equivalent on the ring alkenyl bond.

Subsequent vinyl

group migration and proton loss would then give 2a. The

bicyclooctadienes

respectively retro

of

are of synthetic interest since they proved

hydroxyphenylethynylketene

Diels-Alder

chemistry

2a,b

reactions.4, 5

hydroxyketenes

3 a and

to be precursors

trimethylsiloxyphenylethynylketene

of

3 b via

This is noteworthy since alkynylketenes are very rare I and the

is virtually unexplored.

The ketenes 3 a , b

were generated when p-

xylene solutions of 2a or 2b and a ketenophile were refluxed (138 °C) for 8 hr.

2765

For example, when

2766

SCHEME 1

J

OCH3 (~. "

Ph.. O Ph Z n C ' 2 ~'~,,,,. J/~OR " T.,~ OCH3 "~Ph CCI4 , ~

~~l ~

i

O

H3CO

OCH3

II

138 ° it

Ph

Ph m

1

2

3 a) R = H b) R = Si(CH3) 3

HsC6~ H

H3CO

C6H5

4 1.1 eq. of N-phenylbenzylidenimine was used as the ketenophile the 2-azetidinone 5 was obtained as a single diastereomer in 72% purified yield: mp, 142-143 °C; ir (CHC13, cm -1) 2218, 1780; 1 H N M R (CDCI3) 7.5-7.0 (m, 15H), 5.07 (s, 1H), 0.30 (s, 9H); anal. C, 75.61; H, 5.90.

When 3b was generated in

the presence of 44 eq. of cyclohexene the cyclobutanone 6 was obtained in 50% purified yield as a single isomer: mp, 83-84 oc; ir (CHCI 3, cm "1) 1785; 1HNMR

(CDC13) 7.45-7.30 (m, 5H), 3.62 (m, 1H),

2.73 (m, 1H), 2.15-1.05 (m, 8H), 0.28 (s, 9H); anal. C, 72.98; H, 7.85. Cycloadditions

of 3 a , b

to alkynes gave very unusual results.

First of all, when either 3a or 3 b

was generated in the presence of 1 eq. of diphenylacetylene (138 °C, cyclopentenediones

p-xylene)

the

respective

7a and 7b were obtained in 34% and 55% yield: 7a; rap, 159-160 o c ; ir (CHC13,

cm -1) 1680, 1625; 1HNMR (CDC13) 8.40 (d, J=3.0 Hz, 2H), 7.77 (s, 1H), 7.6-7.2 (m, 13H); anal. C, 85.50; H, 4.49; 7b; mp, 196-197 oc; ir (CHC13, cm "1) 1725, 1690; 1HNMR 7.45-7.22 (m, 13H), 6.94 (m, Hz, 2H), 0.21 (s, 9H); anal. C, 79.18; H, 5.65. well as upon their 13CNMR

The structures of 7a and 7b are based upon the above spectral data as and mass spectral properties.

The

formation of 7 a , b

presents an

interesting mechanistic problem for which there is a precedent. 6,7 That is, analogous products are observed thermolyzed conjugated

when at

4-phenylethynyl-4-hydroxy(or 138

ketene

° C in refluxing p-xylene. 9

(X=Y=OCH3)

trimethylsiloxy)-2,3-dimethoxycyclobutenone This

which undergoes

reaction

is envisaged

ring closure

to proceed

was

via the

to a zwitterionic or diradical

2767

SCHEME 2

H

O~F- IN"C6H5 (CH3) 3 SiO ~

Call 5 m

O

C6H5 H OSi(CH3)3

7

J

I|

C6H5

6

5

Cell5.

U

~

C6Hs,,,,,,~

CH3~

C6H5

C6H5

C2H50"- ~ "Si(CH3)3 O

a) R=H b) R=Si(CH3) 3

R

0 7

X.

O

C6H5

/

o

O

~0

y~ . ~ ~

C6H5

OR +0. Si(CH3)3

10

9 intermediate. 7a,b

Subsequent migration of R (H or Si(CH3)3) then

are envisaged to arise from 9 ( X = Y = C 6 H 5 )

gives the product.

opening of the initially formed 2+2 cycloadducts of the ketenes 3 a , b the

corresponding

diphenylacetylene Finally,

we

cyclobutenones.

constitutes report

the

the

To

our

first example

In analogy,

which, in turn, results from electrocyclic knowledge,

of a hydroxyketene

formation of the benzoquinone

8

with

the

diphenylacetylene,

cycloaddition

of

3a

ring i.e. to

cycloaddition.

(45% isolated yield) when 3 b

was

generated in the presence of 10 eq of ethoxypropyne: yellow oil; ir (CHC13, cm -1) 1650; 1 H N M R (CDC13) -0.09

7.41-7.35 (m,

(s,

9H);

transformation. thermolysis conjugated

3H), 7.15-7.09 (m, 2H) 4.30 (q, J=7.0 Hz, 2H), 1.96 (s, 3H)

exact

mass,

314.1314. 8

Specifically, of

ketene

some

similar

ring

Here

expansions

an

analogy

have

helps

recently

4 - a l k y n y l - 4 - t r i m e t h y l s i l o x y c y c l o b u t e n o n e s . 6,7

proceeds

to

a

zwitterionic

products upon transfer of the trimethylsityl group. Y=OC2H5)

again,

intermediate

and

1.39 (t, J=7.0 Hz, 3H), explain

been Here,

subsequently

this

reported

unusual for

the

the

ring

opened

to

the

quinone

In the case at hand, the ketene 9 ( X = C H 3,

proceeds to 10 followed by trimethylsilyl migration to give 8.

This mechanism was

2768

further

documented

sequential

when

treatment

of

the

cyclobutenone

procursor

of

3-ethoxy-4-methylcyclobutenone

and chlorotrimethylsilane and subjected to thermolysis

9

was

with

independently

prepared

1-1ithio-2-phenylethyne

by

(-78 o )

in refluxing p-xylene (138 °C) for 0.5 hr.

This gave the same reaction products as were formed in the cycloaddition itself. 8 AKNOWLEDGEMENT

The authors wish to thank the National Institutes of Health (GM-36312) and

the Petroleum Research Foundation for financial support of this work. REFERENCES AND NOTES I. Moore, H. W.; Chow, K.; Nguyen, N. V. J. Org. Chem. 1987, 52, 1315. 2. West, K. F.; Wriede, U.; Fernandez, M.; Chow, K.; Moore, H. W. J. Org. Chem. 1987, 52, 2537. 3.

We are grateful to Drs. Shoumo Chang and Joseph Ziller for respectively NMR and X-ray

analyses 4.

studies.

Compound 2 b

was readily obtained as a colorless oil (60%) from 2 a

butyllithium in THF at -78 ° followed by properties 5.

the addition of 2.8 eq of TMSCI.

upon

treatment

with

Its spectral and analytical

are in agreement with its structure.

For a review of the synthetic use of the retro Diels-Alder reaction see: Desimoni, G.; Tacconi, G.;

Barco, A.; Pollini, G. P. Natural Products Synthesis

Through Pericyclic Reactions" ACS

Monograph

180, 1983, American Chemical Society, Washington D.C. 6. Karlsson, J. O.; Nguyen, N. V.; Foland, L. D.; Moore, H. W. J. Am. Chem. Soc. 1985, 107, 3392. 7. Peril, S. T.; Foland, L. D.; Decker, O. H. W.; Moore, H. W. J. Org. Chem. 1986, 51, 3067. 8.

Another product

analogous to 7.

is also

formed

and

is believed

to be the corresponding

The ratio of the quinone to this product in the crude

together they constitute a 47% yield. in the cyclobutenone thermolysis.

cyclopentendione

reaction mixture is

3:2 and

These products in an identical relative ratio are also realized Here, the relative ratio of the two products remained constant

and their combined yield was 58% after 0.5 h and 72% after 19 h.

(Received in USA 28 January 1988)