Oxetanes alkynylation

The photochemistry of more complex and highly substituted alkenic partners has been studied. In 1978, Hartmann and coworkers reported the photocycloaddition of 4-oxazolin-2-one with acetone, used as a photosensitizer in the reaction of 4-oxazolin-2-one with alkenyl and alkynyl partners, to form oxe-tane (44). Recently, Scharf has described the photochemistry of 3-acetyl-2,3-dihydio-2,2-dimcthyloxa-zole (45). Irradiation of (45) in the presence of acetophenone produced the oxetane (46) with the phenyl group endo (17%), in addition to 21% of a ring-opened derivative. The stereoselectivity is in agreement with the high exo carboxyl selectivity observed in the photocycloaddition of methyl phe-nylglyoxylate with 2,2-dimethyl-1,3-dioxole to produce oxetane (47). 

An example in which the chemoselective photoaddition of carbonyl compounds to a conjugated enyne becomes an important issue has been reported by Carless. A solution of 2-methylbut-l-en-3-yne and benzophenone was irradiated to form in 58% yield the oxetane (76), which was converted under acid catalysis to conjugated enyne (77). Surely the synthetic utility of such a process, as well as further applications of alkynyl-carbonyl photocycloadditions, will be forthcoming. 

Alkynylation of oxiranes or oxetanes with lithium alkynides is effectively carried out in the presence of BF3 OEt2 at -78 C. The use of BF3 gives better results than TiCU, SnCU or AlCU. The reaction takes place stereospecifically with anti opening, and the attack generally occurs at the less hindered site. Several functional groups such as halogens, acetals or certain esters survive the reaction conditions (Scheme 16). 100-102... 

U.v. irradiation of the unsaturated A-seco-5-ketone (324) gave none of the expected oxetan (325), but instead produced the cyclobutanols (327) as major products, along with a little of the B-seco decarbonylation product (328). Cyclobutanol formation proceeds through hydrogen transfer from C-2 to the carbonyl oxygen, which is followed by cyclization of the 2,5-biradical (326). Similar reactions occur with the alkynyl-ketone (329) and with the saturated analogue (330). ... 

Reactions at a jSy-alkynyl function in an acetylenic sulphone (140) give products indicating the influence of the sulphonyl group, e.g. the formation of the vinyl sulphone (141), which gives j8-keto-sulphones through cleavage of the oxetan ring at different points on acid hydrolysis. 

In the presence of catalytic amount of Cu(NCMe)4NTf2 (4 mol %), 4-methoxypyridine A -oxide (2 equiv) promotes the oxidative cyclization of alkynyl oxiranes and oxetanes (eqs 15 and 16). The mild reaction conditions tolerate a breath of functional groups such as distal epoxides, alkynes, ethers, acetamide, tosylamide, and aryl halides. 

Alkynyl oxiranes and oxetanes in the presence of a Cu(I) catalyst and a pyridine oxide give five- or six-membered a,j0-unsaturated lactones or dihydrofuranaldehydes in an oxidative cyclization via an allenyloxypyridinium intermediate (Scheme 100). ... 

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