Vinyl ether formation

Clearly, much more information is needed about the behaviour of these two monomers and oxacyclobutane is perhaps the more favorable for further study because of its clearer catalyst-co-catalyst relationship and the absence of vinyl ether formation. The particular information needed about oxacyclobutane reactions at the present time are (a) knowledge of the fate of the catalyst, (b) viscosity-molecular weight relationship, (c) much more information about the variation of molecular weight with the reaction variables, and (d) information about the reaction of both monomer and polymer with oxonium ions and about the ease of formation of oxonium ions by oxacyclobutane. 

In the synthesis of an oxepine from 2,3,4-tri-O-benzyl-D-xylose, l,6-anhydro-3,4,5-tri-0-benzyl-2-deoxy-D-xylosept-l-enitol, the three-step sequence (Wittig olefination, vinyl ether formation, and RCM) was used <2004CAR1163>. 

Figure 1. Quinone methide and vinyl ether formation from free-phenolic lignin units.

Figure 9. Partial spectra illustrating vinyl ether formation in lignin a. acety-lated / -guaiacyl vinyl ether III b.140 °C control c. DEPT spectrum of 140 °C control d. guaiacyl model/lignin product prior to chromatographic separation e. DEPT spectrum of 50 °C control.

Mandai, Saito and cowoikers recently described a new synthesis of isocaibacyclin, which features a crucial one-pot, three-step transformation tandem tertiary allyl vinyl ether formation, Claisen rearrangement, and ene cyclization led from alcohol (57) directly to bicyclo[3.3.0]octane (59 heme 10). Clearly, due to improvements in the preparation of Ae allyl vinyl ether moiety, there is a trend even in the classical Qaisen rearrangement to taclde more complex structural challenges successfully. 

How do reduction reactions that lead to a-CH2-groups affect pulping efficiencies Several typical (P-aryl ether) cleavage pathways would be blocked, which would have a negative effect on efficient delignification. However, because quinone methide formation would be prohibited, undesirable vinyl ether formation and condensation reactions would also be blocked. Bulk-phase fragmentation reactions, involving a... 

One of the difficulties in establishing the detailed chemistry of initial phase pulping is the dominance of the delignification rate by the first step in the process— formation of a quinone methide intermediate. The rates of subsequent steps are difficult to determine however, the rates of these steps are critical to the partitioning of the QMs toward fragmentation processes (p-aryl ether cleavage) as opposed to undesirable competing reactions, such as vinyl ether formation and condensation reactions [57]. 

Mid-Infrared Monitoring Applications during Development of the Vinyl Ether Formation Step in the Preparation of Aprepitant (Emend )... 

Originally the monomer in Fig. 9 was prepared by DuPont by the synthetic scheme shown in Fig. 12 [33]. Thermolysis of the acyl fluoride in Fig. 12 did not give a desired monomer but gave a cyclo compound. In order to prevent the cyclization, a new synthetic route was developed as shown in Fig. 13, which was applied to the synthesis of Dow membranes [34]. A chlorine atom was introduced to the acyl fluoride to improve the selectivity of vinyl ether formation. The Dow membrane was also developed for brine electrolysis, but was not commercialized probably because of its high cost. Difficulty in the preparation of the acyl fluoride in Fig. 13 is one of the causes. Recently, new synthetic processes for the short side chain monomer were developed, as represented in Fig. 14. 

The reaction of secondary aUylic alcohols bearing trans-l,2-disubstituted olefins proceeded smoothly to furnish the desired aldehydes in good yields (Table 3.1.6, entry 1). Geraniols, which form quartemary carbon centers, rearranged in moderate yield because of low reactivity of vinyl ethers formation. And the one-pot rearrangement of styrene derivatives proceeded in low yield due to preparation of deconjugated products (entries 2—4). 

Silyl Vinyl Ethers. Enolizable ketones (16) are readily transformed to silyl vinyl ethers (17) by the action of disilane and a catalytic amount of sodium in HMPA (eq 8). The regioselec-tivity of the silyl vinyl ether formation depends on the structure of the ketones. 

Yamashita, A. and Scahill, T.A. (1982) Reaction of aryl chromium carbene complexes with ethyl propiolate. A versatile vinyl ether formation. Tetrahedron Letters, 23, 3765-3768. 

The ketal connectors which were exploited earlier in O-glycosylations proved equally efficient in the synthesis of C-linked disaccharides as shown in Scheme 32 [61]. With acetate 123 a two-step sequence via vinyl ether formation with Tehbe s reagent and acetalization with alcohol 117 produced the linked sugars 124. Upon tin hydride-promoted cyclization and removal of the acetal tether the a-D-Man(l 4)-d-G1c C-disaccharide 125 was obtained in 35% yield. 

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