FUSION TO THREE-MEMBERED RINGS

Unlike for oxirane fusion to three-membered rings, oxiranes fused to four-membered rings (5-oxabicyclo[2.1.0]pen-tanes, cyclobutene oxides) are well known. 

The fusion of a three-membered ring onto a six-membered ring has much more serious limitations. A three-membered ring must be planar, so it will distort the ring it is being fused to, and this restricts stereochemical possibilities. For example, epoxycyclohexane... 

While 1 is formed by loss of HC1 from bicycloheptene 47, the presence of such angular rather than linear three-membered ring fusion (with respect to the double bond) is not always compatible with cycloproparene formation. Thus tricycle 58, with a tetrasubstitut-ed double bond, fails to yield 35 while its disubstituted isomer does give the desired product, but only in low (4-10%) yield (equation 11)61 62. Similar problems have been... 

A detailed examination of the IR and Raman spectra of thiiranes and thiirenes has appeared in previous reviews <83HC(42/l)333,84CHEC-I(7)131>. The absorption frequencies observed for thiiranes and thiirenes tend to be characteristic for the three-membered ring and are not significantly shifted upon its fusion to carbocycles. However, the S—O bond stretch occurs at 1115 cm-1 for fused ring thiirene oxides but is observed at 1060 cm-1 in 2,3-diphenylthiirene-l-oxide. 

Is the cyclohexane in a chair conformation at all Actually, it is not. The geometry of the three-membered ring forces the cyclohexane ring bonds associated with it and four of the cyclohexane ring carbons to all be coplanar. Only the two cyclohexane ring carbons farthest from the ring fusion are movable. 

Fusion of a three-membered ring to thiophene makes it even more unstable. Evidence has been presented for the formation of cyclopropa[c]thiophene (194) <90TL1315>. Thus dehydrobromination (KOBu ) of (193) in the presence of isobenzofuran gave the adduct (19S) in 57 /o yield (Scheme 33). 

The diazo compound decomposes to gaseous nitrogen and a carbene under catalysis by Cu(Il). Insertion into the exposed alkene gives the three-membered ring. The stereochemistry partly comes from the tether —the hnkage between the carbene and the rest of the molecule that delivers the carbene to the bottom face of the alkene. The rest comes from the inevitable cis fusion between the five- and three-membered rings. 

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