Alkenes, homoallylic stereochemistry

Furthei-more, the cyclization of the iododiene 225 affords the si.x-membered product 228. In this case too, complete inversion of the alkene stereochemistry is observed. The (Z)-allylic alcohol 229 is not the product. Therefore, the cyclization cannot be explained by a simple endo mode cyclization to form 229. This cyclization is explained by a sequence of (i) e.vo-mode carbopallada-tion to form the intermediate 226, (ii) cydopropanation to form 227. and (iii) cyclopropylcarbinyl to homoallyl rearrangement to afford the (F3-allylic alcohol 228[166]. (For further examples of cydopropanation and endo versus e o cyclization. see Section 1.1.2.2.)... 

The Pd-catalyzed hydrogenolysis of vinyloxiranes with formate affords homoallyl alcohols, rather than allylic alcohols regioselectively. The reaction is stereospecific and proceeds by inversion of the stereochemistry of the C—O bond[394,395]. The stereochemistry of the products is controlled by the geometry of the alkene group in vinyloxiranes. The stereoselective formation of stereoisomers of the syn hydroxy group in 630 and the ami in 632 from the ( )-epoxide 629 and the (Z)-epoxide 631 respectively is an example. 

Allylboron compounds have proven to be an exceedingly useful class of allylmetal reagents for the stereoselective synthesis of homoallylic alcohols via reactions with carbonyl compounds, especially aldehydes1. The reactions of allylboron compounds and aldehydes proceed by way of cyclic transition states with predictable transmission of olefinic stereochemistry to anti (from L-alkene precursors) or syn (from Z-alkene precursors) relationships about the newly formed carbon-carbon bond. This stereochemical feature, classified as simple diastereoselection, is general for Type I allylorganometallicslb. 

In the case of tri-substituted alkenes, the 1,3-syn products are formed in moderate to high diastereoselectivities (Table 21.10, entries 6—12). The stereochemistry of hydrogenation of homoallylic alcohols with a trisubstituted olefin unit is governed by the stereochemistry of the homoallylic hydroxy group, the stereogenic center at the allyl position, and the geometry of the double bond (Scheme 21.4). In entries 8 to 10 of Table 21.10, the product of 1,3-syn structure is formed in more than 90% d.e. with a cationic rhodium catalyst. The stereochemistry of the products in entries 10 to 12 shows that it is the stereogenic center at the allylic position which dictates the sense of asymmetric induction... 

The cyclization of carbamate derivatives of unsaturated amines has proven synthetically useful. Cyclizations of carbamates of allylamines containing a terminal vinyl group give oxazolidinone products (equation 60 and Table 17, entries 1 and 2).99,161 Bromocyclizations of systems with a di- or tri-sub-stituted alkene often give mixtures of oxazolidinones and tetrahydrooxazinones,163 while cyclization of an A -cinnamyl carbamate with phenylsulfenyl chloride gave only the oxazolidinone product.163b,163c The stereochemistry of the cyclization of primary carbamates of either allylic or homoallylic amines is low... 

A stereochemical study of the synthesis of unsaturated 1,4-aminoalcohols via the reaction of unsaturated 1,4-alkoxyalcohols with chorosulfonyl isocyanate revealed a competition between an retentive mechanism and an SnI racemization mechanism, with the latter having a greater proportion with systems where the carbocation intermediate is more stable.254 An interrupted Nazarov reaction was observed, in which a nonconjugated alkene held near the dienone nucleus undergoes intramolecular trapping of the Nazarov cyclopentenyl cation intermediate.255 Cholesterol couples to 6-chloropurine under the conditions of the Mitsunobu reaction the stereochemistry and structural diversity of the products indicate that a homoallylic carbocation derived from cholesterol is the key intermediate.256 l-Siloxy-l,5-diynes undergo a Brpnsted acid-promoted 5-endo-dig cyclization with a ketenium ion and a vinyl cation proposed as intermediates.257... 

A Lewis-basic substituent in the alkene can also promote addition of a Grignard reagent to a double or triple bond. Allyl, benzyl, and t-butyl Grignard reagents add readily to allylic and homoallylic alcohols and alkynols (equation 49). A magnesium alkoxide, formed initially, apparently assists intramolecularly in the addition. There appear to be multiple mechanistic pathways, with different stereochemistries. OR and NR2 groups also activate addition. 

Considerably less work has been carried out with more heavily substituted acyclic alkenes for all practical purposes these cannot at present be considered useful substrates. A significant exception exists in the reactivity of the homoallylically derivatized systems of Krafft again, greatly improved yields are observed along with impressive regioselectivity but no diastereoselectivity (equations 21 and 22). It is not known at what stage stereochemistry at the saturated a-carbon is lost. ... 

Note that the aldehyde approaches the alkene from the direction anti to the silicon atom. Therefore, when a chiral allylsilane or allylstannane with a substituent in the a-position is used, chirality transfer takes place, to generate the homoallylic alcohol with essentially no loss in enantiomeric purity. For example, reaction of the aldehyde 157 with the chiral allylsilane 158, using boron trifluoride etherate as the catalyst, gave predominantly the syn product 159 (1.151). The absolute stereochemistry can be determined by using a model in which the hydrogen atom on the a-carbon of the allylsilane eclipses the alkene (the so-called inside hydrogen effect ) in order to minimize steric interactions (1.152). 

The asymmetric epoxidation of all four isomeric allylic-homoallylic alcohols of the type 26 and the subsequent hydride reduction of each epoxide to both possible dideoxyheptitols has been reported. " Only three isomers of 26 undergo a diastereoselective epoxidation and it was concluded that the direction of epoxidation for E-alkenes was controlled by the chirality of the allylic alcohol, whereas for Z-configurated olefins the relative stereochemistry between the two alcohols is important. 

Normally, the most basic, and therefore the most highly alkyl-substituled alkene reacts first, but the vanadium catalyst shows strong directing effects that allow the catalyst to overcome the usual selectivity order if an allylic or homoallylic -OH group is present (e.g., Eq. 14.47). In cyclic compounds the stereochemistry of the final epoxide is determined by the directing effect of the -OH group to which the catalyst binds (Eq. 14.48). Peracids tend to give the other isomer of the product, by a simple steric effect. 

The reaction of terminal allyl alcohols proceeds in a 5-endo fashion to give five-membered ring compounds regioselectively and stereoselectively. Subsequent oxidation affords 2,3-5 y -l,3-diols preferentially, regardless of the nature of the catalyst (eq 1). The stereoselectivity increases with increased bulkiness of the al-lylic substituent and the nature of the alkene substituent (see below). 5-Exo type cyclization occurs with homoallyl alcohols to form five-membered heterocycles and 1,3-diols after oxidation. Two chiral centers are produced in this reaction. The 2,3-relationship (anti) is controlled by the allylic substituent, while the 3,4-relationship is determined by the stereochemistry of the alkene the hydrosilation occurs by cis addition of Si-H to the alkene (eq 2). ... 

The alkene geometry of the homoallylic alcohol dictates the C3 stereochemistry relative to the nucleophilic trap in Prins cyclizations (Scheme 34) [70]. For example, F-alkene 122 undergoes facile Prins cyclization through a chair-Uke transition state where the C3 substiment is in an equatorial position. Subsequent equatorial nucleophilic attack gives rise to 3,4-fra j THP 125. In contrast, Z-alkene 126 can undergo Prins cyclization leading to 3,4-cis product 129 however, diaxial interactions from the axially disposed substituent often suppress this pathway in favor of an envelope transition state leading to THF 130 [71]. 

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