Protolytic Opening of Two Diastereoisomeric Cyclopropanols

Cob(I) alamin ( 1(I) )‐catalyzed reduction of the aldehyde 2 led to the two crystalline cyclopropanols 3 and 4 (see Scheme 2 ). The protolytic ring‐opening starting from 3 produced the saturated aldehydes 6 and 7;8 was formed in traces only (see Scheme 3 ). The protolysis starting from 3 led, therefore, mainly to retention of configuration at the spiro C‐atom ( 7 ); ring‐opening with inversion was observed in traces only ( 8 ). Starting from 4 , the protolysis produced 9 and 7 ; the absence of 8 showed this protolysis to proceed 9 and 7 ; the absence of 8 showed this protolysis to proceed exclusively with inversion of configuration at the spiro center. Of the p ‐bromobenzoate 5 ( cf. Scheme 2 ) the structure has been determined by X‐ray analysis.