Oxetane cyclization

An unusual oxetane cyclization of compound 170 with the participation of the neighboring carbonyl group to yield bicyclic acetal 171 can be induced by triflic acid716 [Eq. (5.266)]. 

The salts of -halo acids cyclize in ionizing media to oxetan-2-ones, as do -diazonium carboxylates (64HC(19-2)787). Thietanes are obtained analogously (Section... 

PATERNO BUCHt 2+2 Cydoadditlon Photochemical 2+2 cyclization ol cartxinyia and oleflns to oxetanes... 

Compared to 1,4-butanediol, which forms tetrahydrofuran byproduct in PBT polymerization, PDO is difficult to cyclize into oxetane because of the high ring strain. Oxetane was not found in the byproduct analysis [23],... 

Acetyl-17-hydroxysteroids on oxidation with IBD lead to the formation of spiro-oxetan-3-ones. The reaction proceeds through an intermediate of the type 31, which undergoes intramolecular cyclization involving participation of the C17-OH group (85S1129) (Scheme 12). 

Oxetane is a colourless liquid, formed by the cyclization of 3-acetoxy-1-chloropropane with potassium hydroxide, or from 1,3-dihydroxy-propane on treatment first with sulfuric acid and then with sodium hydroxide (Scheme 8.12). 

An intramolecular substitution of trimethylamine fix>m 17 gave a bicyclic oxetane 18 in a diastereoselective process <98MI2185>. A [2+2]cycloaddition of 2,2,4,5-tetrasubstituted 2,3-dihydrofuran to aryl aldehydes gave the bicyclic oxetane 19 <98JCS(P1)3261>. 2,2-Disubstituted-3-bromooxetane was obtained by a 4-endo-tcig cyclization process of 3,3 -disubstituted allyl alcohol in the presence of bis(collidine)bromine hexafluorophosphate <99JOC81>. 

In a new method of ring enlargement of oxiranes to oxetanes, the intramolecular Williamson reaction is the essential last step. Reaction of an oxirane with selenomethyl-lithium reagents, followed by bromination, gives y-bromo alcohols, which can be cyclized by base to oxetanes. Since the oxiranes can be prepared readily from ketones and the selenomethyllithium reagent, this is also a synthesis of oxetanes from ketones (80TL585). 

A rapid one-pot method for converting 1,3-diols into oxetanes by the intramolecular Williamson reaction has recently been described. The monolithium salt is generated by treatment of the diol with one equivalent of butyllithium in cold THF, followed by addition of one equivalent of tosyl chloride to give a monotosylate, which is cyclized by addition of a second equivalent of butyllithium (equation 83). Yields of 70-90% are reported for a variety of alkyl- and aryl-substituted oxetanes (81S550). Another simple method for converting 1,3-diols into oxetanes consists of converting them to cyclic carbonate esters by ester... 

The above synthetic methods for oxetane all involve formation of a new C—O bond. Cyclization by formation of a new C—C bond has been applied with compounds having benzylic or alkylic CH groups. Recent examples of this type of ring closure are the rearrangement of trans- 2,3-epoxycyclohexyl allyl ether by means of s-butyllithium and the dehydrochlorination of a-cyanobenzyl 2-chloroethyl ether with aqueous base and phase transfer catalyst (equation 86). Both reactions probably involve carbanion intermediates (76TL2115, 75MIP51300). 

Bond formation a to the heteroatom is a common path to azetidines, oxetanes, thietanes and many oxo derivatives. Attack by the heteroatom on the 7-position is disfavored by the entropy factor the reaction rates for cyclization are about 100 times less than for attack on (3-positions. However, exounsaturation can change this relation drastically in dimethyl sulfoxide at 50°C, (3-bromopropionate ion cyclizes about 250 times more rapidly than bromoacetate. 

Diones are normally synthesized from /3-hydroxy acids in two steps first, conversion into carbamates by reaction with sodium cyanate, and then cyclization with thionyl chloride (Scheme 103) (54JCS839). Alternative preparations utilize oxetanes, which may be combined either with isocyanates in the presence of boron trifluoride (68JAP6808278) or with S-alkylthioureas (Scheme 104) (69ZOR1844). In the last example the initial products are imines (224) which may readily be hydrolyzed to the required diones. Similar methods can be applied to the synthesis of tetrahydro-l,3-thiazine-2,4-diones, and, for instance, the 4-oxo-2-thioxo derivative (225) is obtained from /3-propiolactone and dithiocarbamic acid (Scheme 105) (48JA1001). 

Cyclofunctionalization of homoallylic alcohols with bis(sym-collidine)iodine(I) perchlorate produces oxetanes in good yield if the 4-exo mode of cyclization is favored electronically by the alkene substitution pattern (Table l).14 Geminal substitution at the carbinol carbon also favors this mode of cyclization (compare entries 2 and 3 with entry 1). Substitution at C-3 leads to reaction only by the 4-exo mode (entry 4), while substitution at C-4 leads to cyclization only via the 5-endo mode (entry 5). 

Bright, Coxon, and Steel697 reported the cyclization of a methallyl carbinol in HSO3F via the intermediacy of a protonated oxetane involving a unique oxygen atom migration [Eq. (5.252)]. 

These diols can be cyclized to tetrahydrofurans. Oxetanes are also cleaved in the same way by lithium and a catalytic amount of LDBB. 

The synthesis of oxetanes by intramolecular cyclization of alcohols includes the stereoselective synthesis of (13) from the... 

Resin-based chemistry has been used to construct 480 from 481 by use of boron trifluoride to mediate both the cyclization and cleavage steps <1999AGE1121 >. When the monosaccharide was bound to the polystyrene resin (Merrifield and MPP type) by an alkylsulfonyl linker 482 and cyclization was mediated by 2- r/-butylimino-2-diethylamino-l,3-dimethylperhydro-l,3,2-diazaphosphorine, the reaction had low stereoselectivity and the products included tricyclic oxetanes and oxiranes <2004EJ04177>. The Mitsunobu reaction was used to obtain intramolecular N-alkylation of 483 and formation of 484 <2005AGE3732>. 

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