Six-membered cyclic allylic

The reaction proceeded with extremely high syn diastereocontrol with five- and six-membered cyclic allylic alcohols with a variety of zinc carbenoids. One particnlarly interesting example of a highly functionalized starting material is in the introduction of the C19 methyl group of taxusin through a highly chemo- and diastereoselective cyclo-propanation (equation 50). ... 

The hydroxyl group-directed epoxidation was utilized by M. Isobe et al. in their total synthesis of 11-deoxytetrodotoxin. ° The six-membered cyclic allylic alcohol was treated with mCPBA in the presence of a phosphate buffer to afford an almost quantitative yield of the desired (3-epoxide. 

A six-membered cyclic allylic carbonate 102 undergoes a palladium-catalyzed decarboxylative C-C bond cleavage to afford dienic carbonyl compound 104 [122]. Decarboxylation of the allylic carbonate moiety provides the driving force for production of the intermediate five-membered hetero-palladacycle 103, from which formal reductive cleavage takes place. 

The Simmons-Smith reaction is influenced by a suitably situated hydroxy group in the alkene substrate. With allylic and homoallylic alcohols or ethers, the rate of the reaction is greatly increased and, in five- and six-membered cyclic allylic alcohols, the product in which the cyclopropane ring is cis to the hydroxy group is formed stereoselectively (4.89). These effects are ascribed to co-ordination of the oxygen atom to the zinc, followed by transfer of methylene to the same face of the adjacent double bond. 

It has been reported that a retro-oxidative cyclization process proceeds in catalytic C-C bond cleavage reactions. For example, a six-membered cyclic allylic carbonate 38 underwent a palladium-catalyzed decarboxylative C-C bond cleaving reaction to afford dienyl aldehyde 40 (Scheme 7.12) [15]. It is proposed that oxidative addition of the allylic carbonate to palladium(O) followed by elimination of carbon dioxide generates the palladacycle 39. Subsequent retro-oxidative cyclization produces the diene and aldehyde functionalities. 

Like the Diels-Alder reaction discussed in Sections 14.4 and 14.5, the Claisen rearrangement reaction takes place through a pericyclic mechanism in which a concerted reorganization of bonding electrons occurs through a six-membered, cyclic transition state. The 6-allyl-2,4-cyclohexadienone intermediate then isomerizes to o-allylpbenol (Figure 18.1). 

The report suggests that the actual reagent may be a pentacoordinate allyl-siliconate, such as CH,CH=CHCH2Si F4Cs+, which reacts with an aldehyde via a six-membered cyclic transition state. 

Early work was focused to establish the preference for exo- vs endo-mode of cyclization. However, the absence of an effective method for generation of alkyl and/or aryl substituted silyl radicals made this task difficult. The reaction of prototype alkanesilane I with thermally generated t-BuO radicals at 145 °C after 4 h afforded a 48 % yield of unreacted starting material and 19 % yield of a six-membered cyclic product (Scheme 6.1) [1]. Moreover, EPR studies of the same reaction recorded the spectra at temperatures between —30 and 0°C, which were identified as the superimposition of two species having allylic-type (2) and six-membered ring (3) structures, respectively [2]. At higher temperatures radical 2 predominates therefore, the low yield detected in the product studies could derive from the extensive t-BuO attack on the allylic hydrogens. 

The smooth allyloboration of alkynes is known to proceed via an allyl rearrangement, probably including a six-membered cyclic transition state. Thermal treatment of the product initiates a second allyloboration step and a vinyloboration thereafter the whole procedure [Eq. (37)] opens a synthesis of boraadamantanes by further reaction steps 76). 

Similar cyclic imines can be synthesized by intramolecular oxidative amination of aminoalkenes, which are less expensive than aminoalkynes. In the presence of catalytic amounts of [RuCl2(CO)3]2/l,3-bis(diphenylphosphino)propane and excess K2C03/allyl acetate, various aminoalkenes 19 possessing substituent(s) f3 to the amino group afforded five- and six-membered cyclic imines 20 in mod-erate-to-excellent yields (Eq. 8) [14]. 

Allylation of aldehydes with allylsilacyclobutanes proceeds spontaneously at 130-160 °C with regio- and stereospecificity (Equation (23)).101 Under the same conditions, allyldimethylphenylsilane is insensitive to aldehydes. The thermal allylation takes place probably via a six-membered cyclic transition state, which would be assisted by the relatively strong Lewis acidity of the strained silyl group. Allylsilacycles derived from allylchlorosilanes by treatment with 1,2-diols, /3-amino-alcohols, and 1,2-diamines are reactive enough for spontaneous allylation at or below room temperature.102 1023 10215 103... 

The intermediate and key species proposed for the reaction in Scheme 3.2c are hypervalent silicates based on the silicon NMR spectra of (Z)-crotyltrichlorosilane in DMF. This hypervalent silicate has sufficient Lewis acidity based on the electron-withdrawing chlorine groups as well as nucleophilicity due to electron donation from the hypervalent silicon atom to the allyl systems, which enables the reaction to proceed smoothly. Thus, the high levels of diastereoselectivity can be explained by a six-membered cyclic transition state (Scheme 3.2d). 

The proposed catalytic cycle35 involves the formation of a six-membered cyclic carbenium ion intermediate D with n-bonded metal. The failure of rearrangement of allylic imidates with a methyl group at the olefinic 2-position is presumably due to the difficulty of forming a tertiary carbon-metal bond (Table 4. entry 5)59. The stereocontrol of the metal-catalyzed rearrangement is different from the thermal one because of er-bonding of palladium or mercury in the transition slate. 

Restricting consideration to sipafacial-suprafacial geometries, two limiting confbimations are possible for the six-membered cyclic transition state, a four-center or chair conformation with staggered allyl units which resembles chair cyclohexane, and a six-center or boat confomnation with... 

The role of the catecholate group and fluoride is to delocalize negative charge and increase the Lewis acidity of the silicon center, which coordinates a carbonyl oxygen to form a hexaeoordinate silicate. The six-membered cyclic transition state in the chair conformation is consistent with high threo and erythro selectivity similar to that of allyl boronates [91]. It is interesting to see the structure-reactivity and structure-selectivity correlation shown in Sch. 53 [92]. 

Allylic anions generated from 2-propenyl-l,3-dithiane and 2-styryl-l,3-dithiane react exclusively at a-carbon atoms either with carbonyl compounds or with three- to six-membered cyclic ethers in the presence of BF3-Et20. ... 

The intramolecular cyclization of Rh(II) carbenoids onto a sulfoxide S atom allows one to obtain stable sulfoxonium six-membered cyclic ylides. Interestingly, even with allylic-type substituents at the S atom, this reaction stops at the stage of ylide formation, the yields attaining 84% (Scheme 28) (88TL6009). 

Cyclic iodo carbonates. Allylic and homoallylic alcohols are converted regio-and stereoselectively into five- and six-membered cyclic iodo carbonates, respectively, by carboxylation of the alkoxide followed by reaction with iodine. These products can be converted into cis-1,2- or 1,3-diols as illustrated.2... 

The oxidation of the terminal double bond catalyzed by PdCl2/CuCl has been utilized in a general synthetis of 1,4- and 1,5-diketones which are important intermediates for five- and six-membered cyclic ketones8. At first, an allyl group is introduced into the a-position of ketones and then the terminal double bond is oxidized with... 

Cyclic allylic alcohols fairly easily cycloadd dichlorocarbene, particularly if generated under chloroform/base/phase-transfer catalyst conditions. Five- and six-membered-ring allylic alcohols form a mixture of cis- and fran.v-isomers, while those of larger rings form only the trans-isomer. In contrast with the phase-transfer catalysis method, dichlorocarbene generated from bromodichloromethyl(phenyl)mercury did not add to cyclohex-3-en-1 -ol, while cyclonon-3-en-l-ol yielded exo-10,10-dichlorobicyclo[7.1.0]decan-2-ol (28 /o), which could not be purified. Examples of dichlorocarbene adducts to cyclic allylic alcohols are presented in Table 19. 

The reaction may occur through a six-membered cyclic transition state 12 that directs the electrophile to the other end of the allylic system. This is bad enough, but worse is to come. 

Though anions of allyl ethers, the extra functionality allows regiospecific heteroatom-directed lithiation 160 in the manner of chapter 7. Hence reaction of 160 with an aldehyde goes through a six-membered cyclic transition state 161 to give the enol derivative 163 and hence the y-hydroxyaldehyde 164. 

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