28. Photochemical transformations. Part XVI. A novel synthesis of lactones

Abstract

The photolysis of N-iodoamides has been shown to provide a general route to y-lactones. The mechanism and stereochemical aspects of the reaction have been investigated. A general reagent for the preparation of N-iodoamides has been found in t-butyl hypochlorite-iodine.Lactonisation is most conveniently effected by photolysis of the amide in presence of excess of iodinating agent (t-butyl hypochlorite or lead tetra-acetate with iodine).THE synthesis of saturated lactones from saturated acids by oxidative procedures has not hitherto been accomplished in high yield or by a general meth0d.l In contrast the oxidative cyclisation of aromatic acids has been well attested.2 I n connection with the interrelationship of sesquiterpenoid lactones3 there was need for a general method for converting saturated acids into y-lactones. The present Paper reports the solution of this problem.A preliminary communication has already appeared.4Our first approach t o the problem was to attempt the generation of acyloxy-radicals by photochemical methods. Since the photolysis of alkyl hypohalites,5 especially hypoiodites,6 is a convenient and efficient procedure for inducing intramolecular attack of derived alkoxy-radicals on non-activated hydrogen, we studied the photolysis of acyl hypoiodites. However, in this system, decarboxylation is so rapid that there is little if any indication of the formation of y-iodo-acids or of y-lactones. In fact, the photolysis of acyl hypoiodites provides a good method for decarb~xylation.~ (11) --+ (111)VIII) with possible competition from the process (IV) --+ (VI). Reduction to practice involves the selection of X such that the radical -CO*X*We next turned to a general proposition which is summarised as (I)

Chemistry of amido radicals: Stereochemical controls in II radical reactions

“…Finally, the facile cyclization of bromide 6 to the hydrobromide of iminolactone 7 and other allied cases (13,22,28) requires a comment. Mechanistically, intramolecular nucleophilic substitutions are comn~only recognized to require a 'The corresponding N-chloro derivatives appear to undergo 1,5-chlorine-hydrogen exchangc reaction; the yields are, however, low (13).…”

“…This has led us to speculate that a II amido radical may react as an N-centered radical (1) as well as an 0-centered radical (3), whereas a B radical will react predominantly as an N-centered species (2 and 4). In view of the well recognized fact that intramolecular hydrogen abstraction generally occurs at the fifth position from the radical center (20)(21)(22), an amido radical may functionalize the alkyl side chain at the 8-or &position depending on the 0-or N-radical character. It is also desirable to establish the preference of hydrogen abstraction from the acyl and the alkyl side chains in anlido radicals since this information may shed light on the electronic configuration of the ground state of arnido radicals.…”

In situ formed tert-butyl hypoiodite as an efficient oxidant for rapid, room temperature, metal-free aromatization of Biginelli 3,4-dihydropyrimidin-2(1H)-ones under basic conditions

“…38 For example, Barton used this species to prepare N-iodoamides 39 and to decarboxylate carboxylic acids. 40 Other authors have used it in the preparation of esters from carboxylic acids and alkyl iodides, 41 and in the iodination of aromatics with strong electron donor substituents.…”

The chromous chloride promoted addition of N-haloamides to olefins. III. Scope and limitations for the synthesis of N-(2-haloalkyl)amides

“…A microanalytical determination of bromine indicated that about 2% of 4-bromopentanoamide could have been formed. Thus lZN-d-Halocarboxamides are known to photochemically rearrange to the 4-halo isomers through a free-radical chain reaction involving an intramolecular hydrogenatom transfer to the amido radical (3,14). For example, N-bromopentanoamide was reported to yield 37% of 4-bromopentanoamide (3).…”