Oxirane, Vinylic

Organoboranes are reactive compounds for cross-coupling[277]. The synthesis of humulene (83) by the intramolecular cross-coupling of allylic bromide with alkenylborane is an example[278]. The reaction of vinyiborane with vinyl-oxirane (425) affords the homoallylic alcohol 426 by 1,2-addition as main products and the allylic alcohol 427 by 1,4-addition as a minor product[279]. Two phenyl groups in sodium tetraphenylborate (428) are used for the coupling with allylic acetate[280] or allyl chloride[33,28l]. 

The isomerization of vinyl- or ethynyl-oxiranes provides a frequently exploited source of dihydrofurans or furans, but analogous conversions of vinylaziridines have not been applied so often. While most of the examples in Scheme 87 entail cleavage of the carbon-heteroatom bond of the original heterocycle, the last two cases exemplify a growing number of such rearrangements in which initial carbon-carbon bond cleavage occurs. 

Due to the abundance of epoxides, they are ideal precursors for the preparation of P-amino alcohols. In one case, ring-opening of 2-methyl-oxirane (18) with methylamine resulted in l-methylamino-propan-2-ol (19), which was transformed to 1,2-dimethyl-aziridine (20) in 30-35% yield using the Wenker protocol. Interestingly, l-amino-3-buten-2-ol sulfate ester (23) was prepared from l-amino-3-buten-2-ol (22, a product of ammonia ring-opening of vinyl epoxide 21) and chlorosulfonic acid. Treatment of sulfate ester 23 with NaOH then led to aziridine 24. ... 

The use of an unsaturated anionic initiator—such as potassium p-vinyl benzoxide—is possible for the ring opening polymerization of oxirane [43]. Although initiation is generally heterogenous, the polymers exhibit the molecular weight expected and a low polydispersity. In this case, the styrene type unsaturation at chain end cannot get involved in the process, as the propagating sites are oxanions. 

The reaction of the phenylsulphinyl allylic lithium a-carbanion 342 with oxiranes was found by Guittet and Julia to give, after rearrangement and desulphurization, dihydroxy-dienes 343427 (equation 197). Demoute and coworkers have described the alkylation reaction of a very sophisticated 2-alkenyl sulphoxide 344 as a part of the total synthesis of a juvenile hormone 345428 (equation 198). Since the allylic sulphoxide carbanion has an ambident character, the alkylation may occur sometimes also at the y-position. This direction of alkylation is observed in the case of acyclic allylic sulphoxide anions 346, and results in the formation of the corresponding allylic sulphoxide 347 and vinylic sulphoxide 348423 (equation 199). 

Vinyl-oxirane werden mit Diboran unter Allyl-Verschiebung zu Allylalkoholen re-duziert1. 

Preformed polymers containing reactive functionalities other than vinyl unsaturation (oxirane, hydroxyl, carboxyl, etc.)... 

B- HCCH involving weak primary hydrogen bonds Z- HCCH have revealed large non-linearities, but with an angle 0 that remains reasonably close to those predicted by the rules. Figure 22 illustrates this result through the experimentally determined geometries for the cases when B is 2,5-dihydrofuran [200], oxirane [201], formaldehyde [202], thiirane [203], and vinyl fluoride [204], On the other hand, as noted in Sect. 3.1.3, both... 

The analogous process involving allylic epoxides is more complex, as issues such as the stereochemistry of substituents on the ring and on the alkene play major roles in determining the course of the reaction [38]. Addition of the Schwartz reagent to the alkene only occurs when an unsubstituted vinyl moiety is present and, in the absence of a Lewis acid, intramolecular attack in an anti fashion leads to cyclopropane formation as the major pathway (Scheme 4.10). cis-Epoxides 13 afford cis-cyclopropyl carbinols, while trans-oxiranes 14 give mixtures of anti-trans and anti-cis isomers. The product of (S-elimi-... 

Sn2 -Addition to vinyloxiranes. Lithium dimethylcuprate reacts with vinyl-oxiranes by a highly arm-selective SN2 addition to provide homoallylic alcohols. Examples ... 

Although the resulting vinylallenes 48 were usually obtained as mixtures of the E and Z isomers, complete stereoselection with regard to the vinylic double bond was achieved in some cases. In addition to enyne acetates, the corresponding oxiranes (e.g. 49) also participate in the 1,5-substitution (Scheme 2.18) and are transformed into synthetically interesting hydroxy-substituted vinylallenes (e.g. 50) [42], Moreover, these transformations can also be conducted under copper catalysis by simultaneous addition of the organolithium compound and the substrate to catalytic amounts of the cuprate (see Section 3.2.3). 

Scheme 8.26. Kinetic resolution of cyclic vinyl oxiranes 44.

Among the monomers given in Scheme 1, several possess sufficient nucleophilicity to undergo cationic polymerization (7,8), namely 2-vinyltetrahydrofiiran 1, the alkenylfiirans 2, the furfurylidene ketones 3, 2-furyl oxiranes 4 and the furfuryl vinyl ethers 6, Moreover, furfural and its 5-methyl derivative can act as comonomers in certain cationic copolymerizations. 

Thus, 2-furfuryl vinyl ether 6a is extremely sensitive to cationic activation (16) because of its very pronounced nucleophilic character, but the polymerization is accompanied by some gel formation due to abundant alkylation of the furan rings pendant to the macromolecules. This structural anomaly is not encountered with the 5-methylated monomer 6b (16) precisely because electrophilic substitutions take place predominantly at C5 and are therefore impossible with this monomer. A similar difference of phenomenolo was observed with the 2-fiiryl oxiranes 4a and 4b (17). 

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