Medium eight-membered lactones

If the choice is between a seven- and an eight-membered lactone, the seven-membered ring 151 is preferred to the medium sized ring as in a recent series of experiments24 using the collidine-Br+ reagent 150. 

Both materials were isolated in approximately 30 % yield. The iodo compound VII/175 was then reduced photochemically with tributyltinhydride to give 15-pentadecanolide [114]. Experiments with different cycloalkanones (five- to eight-membered) and different lengths of the side chain (two and three methylene groups) showed that this reaction can be used for the synthesis of several medium sized lactones [75] [80] [114]. Under similar reaction conditions, lactols can also undergo ring expansion reaction [115]. The substrates (steroidal lac-... 

The formation of the twelve-membered lactone, IX/52, from the eight-mem-bered ketone, IX/51, prepared by alkylation of the aldehyde, IX/50, can be explained as an enlargement of a medium-sized ring. The driving force is probably the resonance stabilisation of the secondary nitro group in base (nitro-nate), compared to the tertiary nitro group in the starting material (Scheme IX/9) [29],... 

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