Alkene derivatives allylic reaction

Treatment of the 1,2-oxazines 52 with carbon monoxide at 1000 psi in the presence of cobalt carbonyl brings about insertion of carbon monoxide to form the 1,3-oxazepines S3 <96TL2713>. A convenient route to P-lactams fused to oxepines is made available by alkene metathesis. Thus reaction of 4-acetoxyazetidin-2-one with ally alcohol in the presence of zinc acetate, followed by iV-allylation of the nitrogen affords the derivative 54 which cyclises by RCM to form the oxazepinone 55 <96CC2231>. The same communication describes a similar synthesis of 1,3-dioxepines. 

An unusual incorporation of iodine derived from the Na(lO ) used in RuClj/aq. Na(10 )/CCl -CH3CN was noted in the oxidation of the terminal alkene 2-allyl-2,5-dichloro-4-morpholino-cyclopent-4-ene-l,3-dione giving the iodohydrin 5 3,7-dichloro-l-P-hydroxy-3 3-iodomethyl-8-morpholino-2-oxabicyclo[3.3.0]-oct-7-en-6-one and its 3a-epimer. The iodine apparently derives from the formation of or T from the lOj" to which IO is reduced after the RuClj/IO " reaction (Fig. 3.21) [236]. 

One of the problems associated with thermal cyclodimerization of alkenes is the elevated temperatures required which often cause the strained cyclobutane derivatives formed to undergo ring opening, resulting in the formation of secondary thermolysis products. This deficiency can be overcome by the use of catalysts (metals Lewis or Bronsted acids) which convert less reactive alkenes to reactive intermediates (metalated alkenes, cations, radical cations) which undergo cycloaddilion more efficiently. Nevertheless, a number of these catalysts can also cause the decomposition of the cyclobutanes formed in the initial reaction. Such catalyzed alkene cycloadditions are limited specifically to allyl cations, strained alkenes such as methylenccyclo-propane and donor-acceptor-substituted alkenes. The milder reaction conditions of the catalyzed process permit the extension of the scope of [2 + 2] cycloadditions to include alkene combinations which would not otherwise react. 

Desulfonylation. Allylic sulfones with a terminal double bond (1) react with tri-n-butyltin hydride (azobisisobutyronitrile initiation or irradiation) to form allyl-tin derivatives (2), which on protonolysis give terminal alkenes (3). The reaction can be conducted without isolation of 2 in 80-87% overall yield. 

When allylic hydrogens are present in the alkene, A-allyl hydrazine derivatives are formed preferentially by the ene reaction (Section 7.6.). Furthermore, the reaction of styrenes, (Ey -phenylpropene, and stilbenes with diazenedicarboxylates (traditionally known as azodicar-boxylates) and triazolediones affords products involving double Diels-Alder or Diels-Alder-ene reactions (Section 7.2.10.3.3.). 

The Simmons-Smith reaction is an efficient and powerful method for synthesizing cyclopropanes from alkenes [43]. Allylic alcohols are reactive and widely used as substrates, whereas a,j8-unsaturated carbonyl compounds are unreactive. In 1988, Ambler and Davies [44] reported the electrophilic addition of methylene to a,/3-unsaturated acyl ligands attached to the chiral-at-metal iron complex. The reaction of the racemic iron complex 60 with diethylzinc and diiodomethane in the presence of ZnCl2 afforded the c/s-cyclopropane derivatives 61a and 61b in 93 % yield in 24 1 ratio (Sch. 24). 

This process occurs for X = O only in two special cases, which have been developed into synthetic methods for alkenes and allylic alcohols. Reaction of BusSnH with 1,2-bis-MDC derivatives gave al-kenes from diols in a process which was independent of the geometry of the starting diol. Examples are the conversions of (84) to (85) and of (86) to (87). 

GpgZrGlg or related zirconocene derivatives are effective catalysts of the carbomagnesation of 1-alkenes and some bi- and tricyclic alkenes under mild reaction conditions (<25°G). The reaction of aliphatic 1-alkenes with EtMgX is highly regioselective (Scheme 3.28), with internal double bonds being unreactive 1-alkenes with functional groups (e.g. OR, OH, SR, NR2, TMS) on the chain react similarly and allylic alcohols and ethers are active substrates. Addition to 1-alkenes with... 

Ene reaction of alkenes and oxygen (with double bond migration) to form allyl Hydroperoxides with double bond migration and derived allyl alcohols. 

The most important of the concerted cycloaddition reactions is the Diels-Alder reaction between a diene and an alkene derivative to form a cyclohexene. The alkene reactant usually has a substiment and is called the dienophile. We discuss this reaction in detail in Section 10.2. Another important type of [2+4] cycloaddition is 1,3-dipolar cycloaddition. These reactions involve heteroatomic systems that have four rr electrons and are electronically analogous to the allyl or propargyl anions. 

Hydroboration of 15 is thought to occur wathe conformation with the allylic hydrogen atom eclipsing the alkene (compare with reactions of allylic derivatives in Section 1.1.8) ... 

As a reactive dienophile ortbo-benzyne also participates in the ene reaction. Thus, alkenes with allylic hydrogen can undergo concerted reaction to give substituted benzenes. However, the yields are rarely good. Cycloaddition of ortho-benzyne to alkynes should in principle give benzocyclobutadienes. Such intermediates are highly unstable and not surprisingly are not isolated. Instead, the products, formed in low yield, derive from further reaction with another molecule of ortho-benzyne or by dimerization (Scheme 7.32). 

A different approach using alkene derivatives involved allylic substitution reactions. Ethyl cyclohex-3-en-l-carboxylate (7.51) reacted with N-bromosuccin-imide to give 7.52.26 This allowed reaction with potassium phthalimide to give 7.53 in an overall yield of 41%. Catalytic hydrogenation and treatment with methylamine led to ds-3-aminocyclohexane-l-carboxylic acid, 7.22.26... 

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