Oxetane reactions

Osmium tetroxide, reaction with alkenes, 235-236 toxicity of, 235 Oxalic add, structure of, 753 Oxaloacetic acid, structure of, 753 Oxetane, reaction with Grignard reagents, 680 Oxidation, 233, 348 alcohols, 623-626 aldehydes, 700-701 aldoses, 992-994 alkenes, 233-236 biological, 625-626 phenols, 631 sulfides, 670 thiols, 668... 

Acetophenone 73.6 Oxetane, reaction inefficient Norbornene dimers — trace of oxetane ... 

Oxetanes, reactions with carbonyl compounds 84KGS291. 

The reaction of alkenyl mercurials with alkenes forms 7r-allylpalladium intermediates by the rearrangement of Pd via the elimination of H—Pd—Cl and its reverse readdition. Further transformations such as trapping with nucleophiles or elimination form conjugated dienes[379]. The 7r-allylpalladium intermediate 418 formed from 3-butenoic acid reacts intramolecularly with carboxylic acid to yield the 7-vinyl-7-laCtone 4I9[380], The /i,7-titisaturated amide 421 is obtained by the reaction of 4-vinyl-2-azetidinone (420) with an organomercur-ial. Similarly homoallylic alcohols are obtained from vinylic oxetanes[381]. 

A-Substituted pyrroles, furans and dialkylthiophenes undergo photosensitized [2 + 2] cycloaddition reactions with carbonyl compounds to give oxetanes. This is illustrated by the addition of furan and benzophenone to give the oxetane (138). The photochemical reaction of pyrroles with aliphatic aldehydes and ketones results in the regiospecific formation of 3-(l-hydroxyalkyl)pyrroles (e.g. 139). The intermediate oxetane undergoes rearrangement under the reaction conditions (79JOC2949). 

Azetidines under analogous reaction conditions to those above result in six-membered ring formation. However, diketene (472), an oxetan-2-one, offers considerable promise for five-membered heterocycle formation. With hydroxylamine the 3-methylisoxazolin-5-one (473) was formed. Phenylhydrazine gave the corresponding 3-methyl-l-phenylpyrazolin-5-one. 

Anions of small heterocyclics are little known. They seem to be involved in some elimination reactions of oxetan-2-ones (80JA3620). Anions of large heterocycles often resemble their acyclic counterparts. However, anion formation can adjust the number of electrons in suitable systems so as to make a system conform to the Hiickel rule, and render it aromatic if flat geometry can be attained. Examples are found in Chapter 5.20. Anion formation in selected large heterocycles can also initiate transannular reactions (see also Section 5.02.7 below). 

Oxetane, 3,3-bis(trimethylaminomethyl)-methane sulfonate X-ray crystal structure, 7, 365 Oxetane, 3-bromo-synthesis, 7, 390 Oxetane, 2-t-butyl-3-methyl-synthesis, 7, 399 Oxetane, 2-chloro-reactions, 7, 390 Oxetane, 3-chloro-synthesis, 7, 390... 

Oxetane, 2-methoxy-3,3-dimethyl-hydrolysis, 7, 379 Oxetane, 2-methyl-alkylative cleavage, 7, 381 conformation, 7, 365 H NMR, 7, 366 methanolysis, 7, 379 molecular dimensions, 7, 365 reactions... 

Oxetane, trans-3-methyl-2-phenyl-conformation, 7, 365 Oxetane, phenyl-reactions... 

Oxetane-2-methanol, t/ireo-a,3,3,4,4-pentamethyl-3,5-dinitrobenzoate X-ray crystal structure, 7, 365 Oxetane-3-methanol, 3-alkyl-reactions... 

Some ketones undergo a cycloaddition reaction with alkenes to form oxetanes ... 

Four-membered heterocycles are easily formed via [2-I-2] cycloaddition reac tions [65] These cycloaddmon reactions normally represent multistep processes with dipolar or biradical intermediates The fact that heterocumulenes, like isocyanates, react with electron-deficient C=X systems is well-known [116] Via this route, (1 lactones are formed on addition of ketene derivatives to hexafluoroacetone [117, 118] The presence of a trifluoromethyl group adjacent to the C=N bond in quinoxalines, 1,4-benzoxazin-2-ones, l,2,4-triazm-5-ones, and l,2,4-tnazin-3,5-diones accelerates [2-I-2] photocycloaddition processes with ketenes and allenes [106] to yield the corresponding azetidine derivatives Starting from olefins, fluonnaied oxetanes are formed thermally and photochemically [119, 120] The reaction of 5//-l,2-azaphospholes with fluonnated ketones leads to [2-i-2j cycloadducts [121] (equation 27)... 

The Patemo-Buchi reaction " is the photo-eatalyzed eJeetrocyeJization of a carbonyl 1 with an alkene 2 to form polysubstituted oxetane ring systems 3. 

The mechanism of the Patemo-Biichi reaction is not well understood, and while a general pathway has been proposed and widely aceepted, it is apparent that it does not represent the full scope of reactions. Biichi originally proposed that the reaction occurred by light catalyzed stimulation of the carbonyl moiety 1 into an excited singlet state 4. Inter-system crossing then led to a triplet state diradical 5 which could be quenched by olefinic radical acceptors. Intermediate diradical 6 has been quenched or trapped by other radical acceptors and is generally felt to be on the reaction path of the large majority of Patemo-Biichi reactions. Diradical 6 then recombines to form product oxetane 3. 

The oxetane functional unit is a rare but occurring group in natural products and appears both as end products as well as synthetic intermediates. Patemo-Buchi reactions can be used to insert oxetanes directly into biologically active compounds, as in the example... 

Patemo-Biichi reaction between 2-methylfuran 31 and an a-hydroxyaldehyde 30 to form the core oxetane. Two of the three stereocenters set in the electrocyclization appear in the final product while the third is selectively reversed with anomeric assistance. 

Given the relatively rare appearance of oxetanes in natural products, the more powerful functionality of the Patemo-Biichi reaction is the ability to set the relative stereochemistry of multiple centers by cracking or otherwise derivitizing the oxetane ring. Schreiber noted that Patemo—Btlchi reactions of furans with aldehydes followed by acidic hydrolysis generated product 37, tantamount to a threo selective Aldol reaction. This process is referred to as photochemical Aldolization . Schreiber uses this selectivity to establish the absolute stereochemistry of the fused tetrahydrofuran core 44 of the natural product asteltoxin. ... 

Bis(3-nitrofurazanoxymethyl)oxetane 221 was synthesized in 52% yield by base-promoted ring closure of the corresponding 3-hydroxy-l-propyl triflate, 219, which is readily available from the diol and triflic anhydride. Oxetane 221 can also be prepared in 74% yield by treatment of the trifurazanyl ether 220 with DBU. Polymerization and copolymerization reactions of oxetane 220 have also been investigated (97MI7) (Scheme 148). 

Preparation of oxetanes by [2- -2]-photocycloaddition reactions of ketones and dienes 98S683. 

Two different alkenes can be brought to reaction to give a [2 -I- 2] cycloaddition product. If one of the reactants is an o, /3-unsaturated ketone 11, this will be easier to bring to an excited state than an ordinary alkene or an enol ether e.g. 12. Consequently the excited carbonyl compound reacts with the ground state enol ether. By a competing reaction pathway, the Patemo-Buchi reaction of the 0, /3-unsaturated ketone may lead to formation of an oxetane, which however shall not be taken into account here ... 

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