Lactams five-membered, hydrolysis

A new route to ( )-trachelanthamidine (6) and ( )-isoretronecanol (5) has been reported by Nossin and Speckamp.12 Cyclization of the ethoxy-lactam (28), which is formed on reduction of the imide (27), led to ring-closure with the acetylene to give, after hydrolysis, a 4 1 mixture of the epimers (29) and (30) (Scheme 7). The preferential formation of the five-membered ring is considered to be due to stabilization of an exocyclic vinyl cationic intermediate by the phenylthio group. Reduction of the separated diastereoisomers (29) and (30) yielded ( )-trachelanthamidine (6) and ( )-isoretronecanol (5), respectively. 

Amino-4-deoxy-2,3-0-isopropylidene-D-erythronic acid (155), obtainable from the lactone derivative 153 by treatment with sodium azide followed by hydrogenation of the azido acid (154), forms the lactam (156), but only on sublimation at 150°, because the elements of water must be removed thermally. Once formed, the sterically favored, five-membered lactam (156) is stable toward hydrolysis by acid, in contrast to the six-membered D-ribonolactam (151), and the isopropylidene group can be removed with acid, to give free 4-amino-4-deoxy-D-erythronolactam. 

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