Rearrangements of cyclobutenones. Conversion of selected 4-allylcyclobutenones to bicyclo[3.2.0]heptenones

Abstract

J . Org. Chem. 1989,54, 6018-6021 Scheme I11 TEST H --Co2Et v OAc c K-PH1 5 HI 1 x=o .c-- fi OAc X =aOAc;PH (i) DiBAH c (ii) Ph3P = CH (CH& QK from 12. The overall yield of 1 from 3 is 27%. As pre- viously described, 35J2 is readily available in enantiomer- ically homogeneous form from cis-l,4-diacetoxycyclo- pentene. The stereochemical outcome a t C13 arising from the coupling of similar substrates under the same conditions is amazing. In the previously described aldols5 leading to 5 and 6 no other stereoisomers were observed. Yet when the ynal 7 is employed 8 is the only product observed! Instead of offering ad hoc interpretations that do not flow from sound experimental observations, we prefer to outline an agenda of questions that must be addressed. Why does a particular substitution type on the aldehyde favor or disfavor silyl transfer? Is silyl transfer fundamental or accessory to the stereochemical outcome? Do the sharply differing results arise from a common transition-state (12) Cf: (a) Deardorff, D. R.; Myles, D. C.; MacFerrin, K . D. Tetra- hedron Lett. 1985, 5615. (b) Deardorff, D. R.; Matthews, A. J.; McMeekin, D. S.; Craney, C. L. Ibid. 1986, 1255. alignment differing in the placement of the R and H group of the aldehyde or do the reaction types differ in overall topography (chair vs boat, synclinal vs antiperiplanar)? Answers to these sorts of questions are not readily obtained but are crucial to illuminating this interesting stereo- chemical finding and extending it to new domains. I n the meantime, we note that the chemistry disclosed here and previously5 provides straightforward access to optically pure prostaglandins with complete control of the con- figuration at either C13 or C15 in any stereochemical sense. Acknowledgment. This research was supported by PHS Grant HL25848. NMR Spectra were obtained through the auspices of the Northeast Regional NSF/ NMR facility at Yale University, which was supported by NSF Chemistry Division Grant CHE 7916210. Supplementary Material Available: Experimental proce- dures and characterization data for all compounds (5 pages). Ordering information is given on any current masthead page. Rearrangements of Cyclobutenones. Conversion of Selected 4-Allylcyclobutenones to Bicycle[ 3.2.0lheptenones Simon L. Xu and Harold W . Moore* Department of Chemistry, University of California, Iruine, California 9271 7 Received October 9. 1989 Summary: 4-Allyl-4-alkoxy(or hydroxy or (trimethyl- sily1)oxy)cyclobutenones are reported to rearrange to bi- cyclo[3.2.0] hept-2-en-7-ones upon thermolysis in refluxing toluene. The synthetic scope and mechanism of this un- usual transformation are discussed. The products are envisaged to arise from an electrocyclic ring opening of the cyclobutenones to the corresponding vinylketenes which then undergo an intramolecular [ 2 21 cycloaddition of the ketene moiety to the nonconjugated allylic double bond. Sir: Selected 4-alkynyl, 4-alkenyl-, and 4-arylcyclo- butenones have recently been shown to undergo facile ring expansion to respectively benzoquinones, hydroquinones, 0 1989 American Chemical Society