Hydroxylamines, addition formic acid

Addition of arylhydroxylamines to electrophilic allenes such as methyl propadienoate or l-methancsulfonyl-l,2-propadiene is another route to 0-vinyl derivatives[2]. The addition step is carried out by forming the salt of the hydroxylamine using NaH and the addition is catalysed with LiO CCFj. The intermediate adducts are cyclized by warming in formic acid. Yields are typically 80% or better. 

Blechert has developed an interesting synthesis of 2-substituted indoles which involves the conjugate addition of V-phenylhydroxylamine salts (or V-phenylnitrones) to electron-deficient allenes, followed by carbanion-accelerated hetero-Cc rearrangement of the Michael adduct. For exanple, addition of the hydroxylamine salt (46) to the allenyl sulfone (47) produces the anion (48), which undergoes rapid 3.3-sigmatropic rearrangement to afford the -keto sulfone (49). Cyclization to the indole proceeds smoothly upon exposure to formic acid (Scheme 3). 

An unusual Schmidt reaction has been observed with ketone (56) <95H(40)223>. When the reaction is carried out using hydroxylamine 0-sulfonic acid/formic acid, the expected ring expansion to the tetrahydropyridinones occurs. With hydrazoic acid, however, the unexpected 1,3-diazepine system is obtained (58). This observation is rationalized by the intermediate (57), formed by addition of hydrazoic acid to the initially formed lactam, which then undergoes a Huisgen rearrangement. 

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