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Chapter 29 Suggested solutions for Chapter 29 241 The mechanism is the usual Lewis acid-catalysed conjugate addition of a silyl enol ether on to an rather like the one we saw in Problem 1. H work-up OMe 0 CO₂Me H CI H Problem 11 Suggest a synthesis for the starting material for this reaction, a mechanism for the reaction and an explanation for the selectivity. 0 0 CHO Purpose of the problem of synthesis, mechanisms, and selectivity - the whole works. solution combination suggests a stable enolate adding to an enone. The enone is and the keto-aldehyde can be made by an unambiguous acylation of cyclohexanone with olizable but electrophilic ethyl formate. CHO CHO HCO₂Et base + 0 base 0 The mechanism for the cyclization is straightforward: the ketone forms an enolate at the methyl and attacks the aldehyde. 0 Ho 0 0 CHO KoH 0 KoH E1cB 0 0 The selectivity is tricky. There is an alternative stable enone that might be the product after formation at the same site but attack on the ketone in the ring. Presumably, the is kinetically controlled and the enolate prefers to attack the more electrophilic 0 attack at 0 aldehyde CHO preferred 0 0 0