Cyclobutane cyclobutanol

That the situation is different for photochemical reactions is indicated by a particularly interesting recent study of some dialkylketones (239). In solution, 5-nonanone, 152, reacts photochemically to yield the cyclobutanol 153 and its isomer 154 in comparable amounts. Within the urea clathrate, however, 153 is the dominant product, with only traces of 154 being formed. The cyclobutanols analogous to 153, that is, having methyl and hydroxyl cis, also predominate in the urea-clathrate-mediated photocyclization of 2-hexanone and 2-undecanone. It might be expected that the bulky cyclobutane derivatives, which almost certainly cannot be crystallized in a urea clathrate, would also not be formed in such a clathrate. There are decomposition pathways (cleavage reaction 0 of the diradical intermediate that occur both in the clathrate and in solution. Nevertheless, the ring closure is a major pathway of reaction even in the clathrate. 

The preparation of cyclobutanols by a homoallyl rearrangement has been described previously (see Houben-Weyl. Vol. 4/4, p 62), e.g. treatment of the but-3-enyl p-toluenesulfonate 1 with potassium methoxide in dioxane yielded the dicyanomethoxycyclobutane derivative 2 (61 %). It is also possible to introduce hydrogen into the cyclobutane product by the use of hydrides, for example, borohydrides. Methoxy- or ethoxy-substituted cyclobutanes are formed with alkoxides, while cyano compounds are obtained with potassium cyanide (Table 1). Electronegative substituents in the 1-position of the starting alkene are necessary for a good result in this preparative method. 

Bauld and coworkers have examined the cation radical cycloadditions of 1,3-dienes with electron-rich alkenes and found that, under photosensitized electron-transfer conditions, [2 -i- 2] cycloaddition is in many cases favored over Diels-Alder addition. Thus, as illustrated in equation (30), 1, T-dicyclopentenyl (186) reacts with p-chloroethyl vinyl ether under electron transfer conditions to afford the cyclobutane adduct (187), which was cleaved to the cyclobutanol (188) in 70% yield upon treatment with n-butyl-iithium. Oxyanion-accelerated VCB rearrangement then provided (189) as a mixture of diastereomers in... 

Since the pioneering observation of Demjanov that both cyclobutylamine and cyclopropylcarbinylamine react with nitrous acid to give a mixture of cyclobutanol and cyclopropylcarbinoP , the facile interconversions which occur among related cyclobutane, cyclopropane and open-chain structures via carbenium ion intermediates have been extensively studied. This question has been discussed in Section III.A for a review concerning the formation, structure and ring contraction of cyclobutyl cations, see Ref. 264. 

Photoreactions of Thymines, etc. - Irradiation at 254 nm of the pyrimidine derivative (153) induces a Norrish Type II hydrogen abstraction from a methyl group of the t-butyl substituent. The resultant 1,4-biradical (153a) undergoes cyclization to afford an unstable cyclobutanol. Elimination of water from this species affords the final product identified as the cyclobutane derivative (154). The structure of this product was verified by X-ray diffraction techniques. The Norrish type II reactivity of the pyrimidine derivative (155) at 254 nm in water follows the analogous path to that observed for (153) and yields the cyclized product (156) in 52 % yield. - °... 

Both cyclobutanones and the tertiary cyclobutanols derived from them behave as intramolecular alkylating reagents towards 0-substituted aromatic rings under 4-TsOH catalysis yielding cyclobuta[c]chromans 23. The use of an equimolar amount of 4-TsOH results in subsequent fission of the cyclobutane ring and the formation of chromenes <04T449>. 

In 1980, Baldwin developed a modification of the de Mayo reaction using dioxinone heterocycles as covalently locked enol tautomers of P-keto esters. Thus intermolecular cycloaddition of 2,2,6-trimethyl-1,3-dioxolenone 133 occurs in good yield using stoichiometric quantities of a variety of alkenes. For example, irradiation of 133 with tetramethylethylene yields the cyclobutane adduct 135 in 90% yield. This adduct is converted to cyclohexenone 138 in two steps. Controlled reduction of 135 with diisobutylaluminum hydride (DIBAL) gives keto aldehyde 137 (after spontaneous loss of acetone from hemiacetal 136 and retro-dXdo cyclobutanol fragmentation), which on exposure to acidic conditions affords cyclohexenone 138 in 76% yield. 

---