Ring cleavage acylative, reductive

Since the alkylation products are usually crystalline, purification by recrystallization provides the pure diastereomers prior to cleavage. Hydride reduction of these purified alkylation products furnishes chiral, nonracemic alcohols 7 with moderate to high ee. Although not yet demonstrated, saponification of the alkylation products should be possible and would not occur in the ring, thereby providing an advantage over the corresponding 3-acyl-2-oxazolidinones (see Section 1.1.1.3.34.2.1.)6. 

Cyclic enol-acetates and a-acetoxy-ketones are converted into keto-esters by electro-oxidative ring-cleavage. Keto-esters are prepared (in good yield) by the reduction of acyl ylides (107) with aluminium amalgam (Scheme 60). ... 

Among the six-membered ring heterocycles, diketopiperazines are most commonly prepared by cyclative cleavage. Indeed, diketopiperazine formation is often observed as an undesirable by-product during peptide synthesis20 and the facile nature of this cyclization makes it an obvious choice for library generation. In an early example from Pfizer,21 a set of 10 immobilized a-amino acids was reductively alkylated with 10 aldehydes, followed by acylation with 10 a-amino acids and cyclization (Fig. 6). By... 

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