Oxiranes, fragmentation

Design and synthesis of analogs of dynemycin, antitumor antibiotic with an-thraquinone, 10-member enediyne, tetrahydropyridine, and oxirane fragments 99EJ01. 

Natural peroxides and pharmacological preparations possessing oxirane fragment 99MI3. 

Allenols undergo a palladium(0)-catalyzed cyclization process to yield highly functionalized vinyl oxiranes in a diastereoselective manner (Equation 77) <1999JA7943>. Initial carbon palladation forms a palladium allyl intermediate that suffers subsequent attack by the internal oxygen nucleophile to generate the oxirane fragment. 

Since the interaction of tt)-epoxysiIanes with oxide surfaces at elevated temperatures exclusively leads to the loss of the epoxy fiinction, additional silylation experiments were carried out under ambient conditions. Fig.4 shows some diffuse reflectance infrared spectra of the silica material In all cases, an additional absorption band around 3046 cm characteristic of the intact oxirane fragment is detected. 

Synthetic studies on structurally novel bioactive nine-membered cyclic enediynes with an annulated oxirane fragment 04YGK226. 

Recent developments in the maytansinoid anti-tumor macroheterocycles with N, (9-heterocyclic and oxirane fragments 04CPB1. 

Progress in the synthesis of fumagillin, a sesquiterpene containing two oxirane fragments 03PAC235. 

These are discussed in (B-71MS4). Oxirane itself shows a strong molecular ion peak and a slightly stronger base peak at mje 29 (CHO ) due to isomerization to ethanal and loss of a methyl radical. Substituted oxiranes tend to show only weak molecular ion peaks, because of rearrangement and fragmentation. 

The thermal fragmentation of unsaturated bicyclic 1,4-peroxides, often readily made from 1,4-dienes (Scheme 84), has become an important route to novel bis(oxiranes) (80T833, 81CRV91). 

The outcomes of intramolecular cyclizations of hydroxy vinylepoxides in more complicated systems can be difficult to predict. In a study of the synthesis of the JKLM ring fragment of dguatoxin, epoxide 44 was prepared and subjected to acid-mediated cydization conditions (Scheme 9.24) [114]. Somewhat surprisingly, the expected oxepane 45 was not formed, but instead a mixture of tetrahydropyran 46 and tetrahydrofuran 47 was obtained, both compounds products of attack of the C6 and C5 benzyl ether oxygens, respectively, on the allylic oxirane position (C3). Repetition of the reaction with dimsylpotassium gave a low yield of the desired 45 along with considerable amounts of tetrahydropyran 48. 

The reaction gave only the rearrangement products 333 and 334, and the side product 335, as expected from the reactivity of alkylidenecyclopropane derivatives (Scheme 49). Compound 333 might arise from the 0-0 bond cleavage followed by the rearrangement of a cyclopropyloxy cation to an oxoethyl cation (Scheme 49, path a). Spiro-hexanone 334 could arise from a different fragmentation of ozonide C-O bond and further cyclopropyloxy-cyclobutanone rearrangement (Scheme 49, path b). Oxirane 335 can eventually derive from the same path b or from other side processes [13b]. 

Synthesis of primidolol (65) can be carried out by a convergent scheme. One branch consists in application of the usual scheme to o-cresol (62) ring opening of the intermediate oxirane with ammonia leads to the primary amine (63). The side chain fragment (64) can be prepared by alkylation of pyrimidone (63) with ethylene dibromide to afford Alkylation of ami noalcohol with halide ... 

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