Photochemie von Aminoketonen. V. Diastereoselektive Synthese von 3‐Aryl‐azetidin‐3‐olen durch Photocyclisierung von Aryl‐α‐amidoalkyl‐ketonen

Abstract

Photochemistry of Aminoketones. V. Diastereoselective Synthesis of 3‐Aryl‐azetidin‐3‐ols via Photocyclization of Aryl‐α‐amidoalkyl‐ketones N‐Acyl‐azetidin‐3‐ols 3 and 4 are formed by diastereoselective cyclization of n,π*‐excited N‐benzyl‐N‐phenacyl‐amides 1 and 2 in ether solution. Dependent on the nature of the p‐and α‐substituents of the phenacyl part and especially of the N‐acylgroups in the amides 1 and 2 competing reactions occur, particularly the NORRISH II cleavage and an oxydative cleavage of the intermediate hydroxybiradicals A. N‐Acylgroups with low energy barrier ΔG≠ of internal rotation favour the diastereoselective photocyclization by intramolecular interaction between the amide oxygen atom and the n,π*‐CO‐group in 1*, 2* and the OH group in the biradicals A, respectively. An intramolecular hydrogen bonding enables the N‐tosyl‐azetidinols 3 to rearrange to the N‐tosylamino‐methyl‐benzhydryl‐ketones 6 by 1‐sensitized photoreaction.

Enantioselective Preparation of (2R)- and (2S)-Azetidine-2-carboxylic Acids

“…Normally, the irradiation of such ketones gives a considerable amount of Norrish-Type-II cleavage products, and often the cleavage dominates the cyclization [16]. On the other hand, it was observed that the introduction of a heteroatom into the side chain of the ketone increases the cyclization yield [8] [17]. This effect can be explained by the conjugation of a heteroatom lone pair with the adjacent radical centre (see B in Fig.…”

“…Compound 5 was prepared in analogy to the procedure described by Fuhrmann et al [8] 1,1,2-trimethylpropyl)silyloxymethyl]-3-methoxy-3-phenylazetidin-3-ol (17 and ent-17, resp.) NaH (80%, 0.14 g, 1 equiv.)…”

“…In continuation of our previous work on stereoselective photocyclization of ketones, we were interested in a photochemical synthesis of azetidine-2-carboxylic acids. Although several photocyclizations of amino ketones giving azetidinols were published [7] [8] nothing is known about an appropriate synthesis of azetidine-2-carboxylic acids. This fact is surprising in view of the cited successful azetidine ring-closure reactions.…”