Other substituted oxetanes

Campbell and Foldi [64] and Penczek and Vansheidt [65] have prepared and polymerized a number of bicyclic oxetanes. In these compounds the substitution on the 3-carbon of the oxetane ring was in the form of a hydrocarbon ring. Penczek and Vansheidt [65] made some kinetic measurements. Their rate data compared with earlier values are shown in Table 4. They point out that the strain in the second ring has no 

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The same process shown in Scheme 88 starting from different 2-substituted oxetanes and using biphenyl as the electron-carrier catalyst under THF reflux has been used to prepare regioselectively substituted primary alcohols. On the other hand, the combination of a DTBB-catalyzed ca 20%) lithiation with triethylaluminium in TFIF at —78 °C has been used for the transformation of strained oxetanes to substituted di- and triquinanes through a rearrangement process . An example is given in Scheme 89 for the transformation of oxetane 299 into the product 302 through radicals 300 and 301. 

The chemistry of methylene-substituted 2-oxetanones, and in particular diketene, has attracted a vast amount of research over the years, so other alkylene-substituted oxetanes and oxetan-2-ones, including methylene oxetanes, are discussed separately in Section 2.05.7.4. Diketene is also known as 4-methylene-2-oxetanone and a notable difference between this and other 2-oxetanones is that it undergoes thermal decomposition by a cycloreversion reaction to give ketene, rather than forming allene and C02. CHEC-II(1996) refers to a series of comprehensive reviews of the chemistry of this compound <1996CHEC-II(1)721>. 

In the PB reaction of unsymmetrically substituted furans with aldehydes, the site-selectivity was reported as quite difficult to control (Scheme 7.13). Thus, a 1.3 1 mixture of oxetanes was formed in the PB reaction of 2-methylfuran with benzalde-hyde. Schreiber and coworkers found that the site-selectivity could be controlled by using bulky substituents at the furan ring [29a], and consequently the less-substituted oxetanes were selectively prepared in the PB reaction (Scheme 7.13). On the other hand, a highly site-selective formation of the more-substituted oxetanes was reported in the PB reaction ofacetylfurans with aromatic aldehydes (Scheme 7.13) [29b]. A high exo-selectivity was also observed in the PB reaction with aldehydes (vide infra). 

Other reactions of oxetanes reported include the asymmetric ring expansion of 2-substituted oxetanes in an enantioselectively catalysed reaction with diazoacetic esters to give 2,3-disubstituted tetrahydrofiirans <01T262I>, and the Lewis acid promoted reactions of 2-methoxy-2-siloxyoxetane with allylsilane <00MI651 >. 

MeOTf has been reported to effect the dealkoxylation of a perfluoroalkyltrialkoxyboronate to generate the corresponding boronic ester, l Conversely, an alkenyl boroxycarbene complex was reacted with MeOTf to remove the borane-based chiral auxiliary yielding a Fischer carbene complex. l l ferf-Amide substituted oxetanes rearranged in anhydrous nitrobenzene at 150 °C with a catalytic amount of MeOTf to produce ester-substituted azetidines (eq 16).i Other acids such as boron trifluoride ether-ate, trifluoromethanesulfonic acid, and benzylthiolanium hexaflu-oroantimonate led to low yields of the desired azetidines. 

Properties have been determined for a series of block copolymers based on poly[3,3-bis(ethoxymethyl)oxetane] and poly [3,3-bis(methoxymethyl)oxetane]- (9-tetrahydrofuran. The block copolymers had properties suggestive of a thermoplastic elastomer (308). POX was a good main chain for a weU-developed smectic Hquid crystalline state when cyano- or fluorine-substituted biphenyls were used as mesogenic groups attached through a four-methylene spacer (309,310). Other side-chain Hquid crystalline polyoxetanes were observed with a spacer-separated azo moiety (311) and with laterally attached mesogenic groups (312). 

Photochemical addition of acetone to cis and trans 1-methoxy-l-butene involves both acetone singlets and triplets.71 Since spin inversion in the 1,4-biradical is relatively slow, predominant loss of stereochemistry is observed in the oxetane derived from triplet acetone. On the other hand, stereochemistry is partially retained when acetone singlets attack the substituted olefin. 

Substitution of alkyl and aryl groups on the oxetane ring, on the other hand, increases the barrier of the puckering vibration. 2-Methyloxetane and 3-methyloxetane are non-planar, existing as mixtures of stable equatorial and axial conformers at room temperature (79JST(56)157>. 

When the reaction is applied to an unsymmetrically substituted alkene, the major photoproduct or products are those arising from the most stable diradical intermediate. This is illustrated in the irradiation of benzaldehyde in 2-methylbut-2-ene in which the principal products are the stereoisomeric oxetanes arising from the same diradical intermediate [Eq. (70)].278 The oxetanes resulting from addition in the alternative sense [Eq. (71)] are minor products, as are other products arising by allylic hydrogen abstraction. 

There is a striking difference between the photochemical reactivity of oc,(3-unsaturated enones and the corresponding ynones. Whereas many cyclic enones undergo [2+2] cycloaddition to alkenes at the C=C double bond of the enone (probably from the triplet nn state) to yield cyclobutanes, acyclic enones easily deactivate radiationless by rotation about the central C-C single bond. Ynones on the other hand behave much more like alkyl-substituted carbonyl compounds and add to (sterically less encumberd) alkenes to yield oxetanes (Sch. 11) [38,39]. The regioselectivity of the Paterno-Biichi reaction is similar to that of aliphatic or aromatic carbonyl compounds with a preference for primary attack at the less substituted carbon atom (e.g., 41 and 42 from the reaction of but-3-in-2-one 40 with... 

The diol is made into an epoxide by an intramolecular substitution reaction that is Sn2 and so goes with inversion. There are two possible rings that could form, depending on which hydroxyl group attacks, but (as you will shortly see) three-membered rings form faster than four-membered ones, and the reaction gives none of the oxetane. The other hydroxyl group can now be protected as a benzyl ether. 

In the photoaddition of acetone and other ketones to 1-, 2- and 1,2-di-methylimidazoles the products sire a-hydroxyalkylimidazoles (153) which are derived from the selective attack of excited carbonyl oxygen at C-5. In the case of 2-methylimidazole the products are the 4-mono- (8%) and 4,5-di- (14.5%) substituted compounds, but imidazole itself does not react. The suggestion that it is not a sufficiently electron-rich substrate is not particularly convincing. The reaction mechanism (Scheme 72) may reflect the greater radicd reactivity at C-5, and the comparative stabilities of the radical intermediates derived from carbonyl attack at this position. Hiickel calculations of radical reactivity indices show that, indeed, C-5 is more reactive, and the radical intermediate at C-5 is more stable than that at C-4, but a concerted cycloaddition could also give rise to the oxetane (152). Such an oxetane can be isolated in the photochemical addition of benzophenone to 1-acetylimidazole. 

In a synthesis which correlates (-)-3-pinene with (+)-hinesol (41 Scheme 15), Magnus started with (+)-nopinone (37) which provided, after several steps, the sulfone (38) as the key compound. Under airline conditions, (38) gives the p-keto sulfone (40) via intramolecular attack of the carbanion of (38) forming the alkoxide (39). Release of ring strain is responsible for the domination of the ft gmentation rather than typical alternatives such as oxetane formation, substitution, or other reaction pathways. Traces of water in the fragmentation step give rise to the formation of carboxylic acid (42). 

The Paterno-Biichi Reaction. One well-known class of photocycloadditions is the Paterno-Buchi reaction in which aldehydes or ketones combine with alkenes to give oxetanes.1108,1109 The excited state of the ketone is n-n, and it is the orbitals of this state which interact with the ground-state orbitals of the alkene. Often it is the triplet state which is involved,1110 but occasionally the singlet state is important. The orientation usually observed for C- and X-substituted alkenes is usually counter-thermodynamic, with the more-substituted atoms bonded to each other.1111,1112... 

The low quantum yield indicates that intermediate complexes and diradicals decay unproduc-tively. In general, Norrish type II photoreactions and other hydrogen abstraction processes must be overcome in order to achieve successful cycloaddition. Only in a few reported cases is intramolecular hydrogen abstraction a serious competitive reaction path. For example, the cycloheptenyl-substituted ketone 5 yields an oxetane 6 in addition to a cyclobutanol derivative 758, whereas the unsaturated cycloheptanone 8 only gives oxetane products 9 and 10 on irradiation59. The main product 10 was converted in two steps to azulene in 25 % overall yield. The reaction sequence 11 - 12 - 13 also demonstrates the high synthetic potential of the intramolecular Paterno-Biichi reaction61. 

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