A-Methyl-Y-butyrolactone

Reaction of cinnamyl alcohol (36) catalyzed by Rh-BINAPHOS gives the product as lactol 37 (1 1 mixture of diastereomers at the anomeric carbon) with high enantioselectivity (88% ee) [94] (Scheme 7.7). The enantiopurity of lactol 37 is determined by oxidizing the lactol to the corresponding lactone 38. In the same manner, homoallyl alcohol (39) is converted to the corresponding a-methyl-y-butyrolactone (42) with 73% ee via lactol 40 [94] (Scheme 7.7). However, the regioselectivity of the reaction is not favorable to the formation of 40, forming achiral 6-lactol 41 as the major product. 

Another example is the carbonylation of allyl carbinol under the conditions of the Reppe reaction [525], yielding a-methyl-y-butyrolactone plus S-valerolactone. 

In this group, tuberostemospironine (28) is the only alkaloid which lacks the a-methyl-y-butyrolactone ring appended to C3 of the pyrrolidine ring A. Croomine (29), stemospironine (30), stemotinine (31), and didehydrocroomine (34) display an opposite stereochemistry at C9 to that found in tuberostemospironine (28), isostemotinine (32), and stemonidine (33). Curiously, stemotinine (31) and isostemotinine (32) have an oxygen bridge between C9a and C6. 

Parvistemonine (42) and didehydroparvistemonine (43) have an Q -methyl-)/-lactone ring positioned at C3. The structures of these alkaloids were established by a combination of spectroscopic methods (16,36,51), and by oxidation of parvistemonine (42) to didehydroparvistemonine (43) with Ag20 (16). The striking differences in almost all of the signals in the C-NMR data (Table XV) of parvistemoline (41) and parvistemonine (42) ranging from less than 1 ppm (Cll, CIS, and Cl6) to 11.1 ppm (CIO), compared to the chemical shift differences observed previously for alkaloids which differ only in the replacement of the carbonyl at C3 by the a-methyl-y-butyrolactone ring, makes the current structural assignment for these alkaloids doubtful. 

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