Lacton‐Bildung durch ringerweiternde Fragmentierung von Epoxycyclodecanon‐Derivaten

Lactones from Epoxycyclodecanone Derivatives by Ring Enlargement Involving Fragmentation Reactions A stereospecific ring‐enlargement reaction of alkyl esters of 2,3‐epoxy‐1‐(3‐hydroxypropyl)‐10‐oxocyclo‐decanecarboxylic‐acid derivatives is described, involving Grob fragmentation of in situ formed hemiacetals. The assignment of the relative configuration of the starting materials was accomplished on the basis of 1 H‐NMR data. The rearrangement of the epoxides 9 and 10 (with cis ‐orientation of the ester group and the epoxide ring, Scheme 1 ) gives the lactone 15 as the single and as the major product, respectively, with ( Z )‐configuration of the newly formed CC bond. A concerted reaction mechanism is assumed. The formation of a small amount of 12 from 10 is probably due to a competitive two‐step carbanion pathway. The reaction of the diastereoisomers 7 and 8 leads to the lactones 11 and 12 , respectively, as the only ring‐enlargement products ( Scheme 1 ), with ( E )‐configuration of the newly formed CC bond. On the basis of our results, we cannot distinguish in this case between a concerted and a two‐step carbanion mechanism. This type of reaction takes place only in the presence of an ester group; no ring enlargement was detected in case of compound 20 ( Scheme 3 ), which is the de(alkoxycarbonyl) derivative of 9 . The eliminative opening of the epoxide ring in the epoxylactone 17 affords 11 as the single product ( Scheme 2 ). A carbanion mechanism was assumed for this reaction.