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A 5- exo -dig radical cyclisation of the bromoamide 34 derived from the enantiopure α-ethynyl substituted amino alcohol 31 led to a 2:1 mixture of β-C3 and α-C3 methyl epimers of the pyrrolidinone 35a – 36a in a combined yield of 73%. Treatment of the homoallylic alcohol 35b , derived from 35a , with OsO 4 –TMEDA, gave a single diastereoisomer of the pyrrolidinone triol 37 , resulting from selective dihydroxylation from the β-face, i.e. syn to the CH 2 OH group of 35b . The pyrrolidinone triol 37 is a potential common precursor, cf. 9 , to the spiro β-lactone pyrrolidinone 8 and the γ-lactone pyrrolidinone 10 ring systems in oxazolomycin A ( 1 ) and neooxazolomycin 2 , respectively. Sequential protection of the 1,2-diol functionality in 37 as the acetonide 39 , and the primary alcohol group in 39 as the SEM ether 41a , followed by methylation of the nitrogen centre in 41a , using NaH–MeI, then gave the selectively protected pyrrolidinone 42 . |
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