Oxetane, conformation

Boge, T. C. Heperle, M. Vander Velde, D. G. Gunn, C. W. Grunerwald, G. L. Georg, G. I. The oxetane conformational lock of paclitaxel structural analysis of D-secopa-clitaxel. Bioorg. Med. Chem. Lett., 1999, 9 3041-3046. 

Boge TC, Hepperle M, Vander Velde DG, Gunn CW, Grunewald GL, Georg GI (1999) The Oxetane Conformational Lock of Paclitaxel Structural Analysis of D-secopaclitaxel. Bioorg Med Chem Lett 9 3041... 

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, 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... 

Stereochemistry can be interpreted in terms of conformation effects in the 1,4-biradical intermediates.199 Vinyl enol ethers and enamides add to aromatic ketones to give 3-substituted oxetanes, usually with the cis isomer preferred.200... 

The origins of the observed conformations of azetidine, oxetane and thielane... 

There are some cases where both types of photocycloaddition take place. For example, cinnamaldehyde and crotonaldehyde yield, upon irradiation with 2-methyl-2-butene, both the oxetane and the cyclobutane products.26 In marked contrast, mesityl oxide, as similar as it would appear to be to crotonaldehyde (Table I), is stable to irradiation in the presence of both isobutylene and isopropanol.37,74 These differences in reactivity of a,/9-unsaturated carbonyl compounds have been attributed to conformational (that is, s-cis or s-trans) differences.74... 

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>. 

Intramolecular photoaddition in cis-5,5,6-trimethylhepta-3,6-dien-2-one (284) takes a different course,296 yielding not the oxetane but the two dihydropyrans (285 and 286). This is in contrast to the inter-molecular cycloaddition of a, jS-unsaturated aldehydes to alkenes which affords only oxetanes, and has been accounted for in terms of diradical intermediates (287 and 288) formed from the s-cis conformation (284) of the dienone. The intermolecular equivalent is thought to occur by addition to the s-trans conformation. 

Molecules of 43c adopt chiral packing (space group P2 ) and a helical molecular conformation, and crystallize in (E,Z) conformation which is unfavorable for the oxetane formation. The solid-state irradiation of 43c was found to give the oxetane 44c and a 3-lactam derivative 45c. The (3-lactam 45c was revealed to be enantiomerically enriched to 88% ee, whereas the other photoproduct 44c was racemic. The occurrence and the mechanism of transformation of 43c to 45c involve hydrogen abstraction by the alkenyl carbon atom. 

When a four-membered heterocycle is cis-fused on the side of another ring, as with syn-44, which we met in chapter 12, cyclisation of the syn-monotosylate 42 in base is very efficient as the, usually unfavourable, conformation 10 is now the only possible one and the nucleophile and electrophile are perfectly arranged 43 for cyclisation. This observation took on a new importance when the anti-cancer compound taxol was discovered as it also has a cis -fused oxetane.6... 

A few theoretical studies of oxetanes and oxetanones have been reported since CHEC-II(1996). Building upon a study of the oxetane- -HCl complex studied by rotational spectroscopy, MP2 calculations were used to investigate the axial and equatorial HCl arrangement, and to try and explain why for oxetane- -HCl only one conformer was observed <2001CPL250, 2002CPL123>. The amine-catalyzed aldol reaction via enamine intermediates has been explored using density functional theory (DFT) (B3LYP/6-31G ) and conductor-like polarizable continuum model... 

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