Prins reaction promoted

All syn-Z fi, 4- 3co-methylene tetrahydropyrans 367 can be readily assembled using a Lewis acid promoted Prins reaction between enol carbamates 368 and aldehydes (Equation 153). The presence of an o-methylene group and the labile carbamate in the product allows for simple transformation into tetrahydropyranones and tetrahydropyranols <2000TL7225, 2001TL8685, 2002JOC8744>. 

This reaction has been extended to the reaction between aldehydes and alkenyl complexes of boranes, silanes, and stannes to give allylic alcohols, " where the reaction with alkenyl complexes of silanes is known as the silyl-Prins reaction. In addition, several Lewis acids-mediated or -promoted Prins reactions have recently been developed, including the application of TMSI, hafnium (IV) bis(perfluorooctanesulfonyl)amides (in fluorous biphase system),iron (III) species," and 2,6-di-f rf-butylphenoxy(difluoro)borane. J... 

Highly functionalized tetrahydropyran-4-ones can be obtained through a silyl enol ether Prins cyclization promoted by a condensation reaction of hydroxy silyl enol ethers with Lewis-acid-activated aldehydes, with high diastereoselectivity (14JOC8733). Spirooxindole tetrahydropyran-4-ones are obtained from oxa-DA reaction of acyclic a,P-unsaturated methylke-tones and isatins mediated by a bifunctional enamine-metal Lewis acid catalyst, in good yields and moderate stereoselectivities (14S1339). 

The acid-promoted Prins reaction between a homoaUyhc alcohol and an aldehyde is a weU-estabhshed synthesis of tetrahydropyrans (Scheme 4) [ 14,15]. While substituted tetrahydropyrans are often assembled by cyclizations forming a C - O bond, the Prins reaction undergoes cyclization by C - C bond formation. The Prins reaction of the silyl-modified substrates [16], which can be regarded as the intramolecular Hosomi-Sakurai reaction, is effectively activated by the allylsilane unit. The stereochemistry of the 2,6-cfs-form produced in the case of the exo-allylsilane unit is elucidated by the 6-membered transition state model, hi the cyclization of the ewdo-aUylsilane substrates, since the silyl group would be fixed on the axial position of the 6-membered transition states, the tetrahydropyrans with both 2,6-cis and fraws-forms can be synthesized (Panek Sect. 3.3.9). This type of cychzation was also... 

Methyl-l-phenylhept-3-yne-2,6-diol undergoes a one-pot three-step cascade reaction with alkyl and aryl aldehydes to provide 2,4-dihydro-lH-benzo[ isochromans.This two-component cascade reaction promoted by Bp3-OEt2 involves an alkynyl-Prins cyclization, Friedel-Crafts arylation, and dehydrative aromatization (130L4070). Other Prins cascade cycliza-tions of 2-arylethylbut-3-en-l-ol with aliphatic and aromatic aldehydes occur in the presence of Sc(OTf)3 andp-TsOH to prepare trans-fused hexa-hydro-1 H-benzo[ isochromans in good yields and excellent selectivity (Scheme 44) (13EJ01993). 

The Prins cyclization has been shown to be a fundamental method for the formation of O-heterocycUc derivatives [11, 12]. Thus, to date, there have been numerous examples on the Prins reaction for the formation of tetrahydropyran units. The mechanism involves, in general, the acid-promoted condensation reaction between a homoaUyl alcohol and an aldehyde in the presence of a Lewis acid leading to tetrahydropyran units (Scheme 1.1). The key intermediate, an oxocarbenium ion 1,5, is generated from a hemiacetal and undergoes 6-endo cyclization to provide a secondary carbocation 1.6 which can be readily trapped by various nucleophiles. 

Parchinsky et al. (2011) reported synthesis of symmetrically and unsymmetrically substituted chiral, racemic 1-azaadamantanes via BF3.0Et2-promoted aza-Prins reaction under microwave irradiation. Reaction of ( )-[(4-methylcyclohex-3-en-l-yl) methyljamine with an equimolar amount of an aldehyde in the presence of 1.0 equiv. of Bp3.0Et2 at ISO C for 1 h gives predominantly bicyclic piperidines whereas in presence of excess amount of the aldehydes, symmetrically substituted 1-azaadamantanes are the major products. The yield of the products ranges from 52-83%. 

A plausible mechanism for this new alkyne aza-Prins cyclization is outlined in Scheme 27. Thus, reaction of the homopropargyl tosyl amine with an aldehyde promoted by ferric halide generates the W-sulfonyl iminium ion. This intermediate evolves to the corresponding piperidine, via the vinyl carbocation. Ah initio theoretical calculations support the proposed mechanism. 

FeX3 was also found to be an excellent promoter in the classical Prins cycliza-tion (Scheme 10, route H), with the observation of a satisfactory reaction between 3-buten-l-ol and several aldehydes, affording the corresponding c/s-4-halo-2-alkyl tetrahydrop3Tans in good yields [Eq. (1) in Scheme 32] [35], In a similar manner, the methodology can be extended to the piperidine synthesis through an aza-Prins cyclization [Eq. (2), Scheme 32] [41],... 

Prins summarizes advances in understanding of the reactions in catalytic hydrodenitrogenation (HDN), which is important in hydroprocessing of fossil fuels. Hydroprocessing is the largest application in industrial catalysis based on the amount of material processed. The chapter addresses the structures of the oxide precursors and the active sulfided forms of catalysts such as Ni-promoted Mo or W on alumina as well as the catalytically active sites. Reaction networks, kinetics, and mechanisms (particularly of C-N bond rupture) in HDN of aliphatic, aromatic, and polycyclic compounds are considered, with an evaluation of the effects of competitive adsorption in mixtures. Phosphate and fluorine promotion enhance the HDN activity of catalysts explanations for the effect of phosphate are summarized, but the function of fluorine remains to be understood. An account of HDN on various metal sulfides and on metals, metal carbides, and metal nitrides concludes this chapter. 

Prins et al. (21) described the lower flammability of poly-bromostyrene relative to that of polystyrene. On the basis of thermal analysis experiments, they suggested that bromine inhibited most of the oxidative chain reactions, and thus the combustion was not supported (vapor-phase mechanism). Khanna and Pearce (16) and Brauman (22) demonstrated that polystyrene could be flame retarded by appropriately modifying its structure with substituents that promote the char yield of the system (condensed-phased mechanism). 

Li et al. reported an early use of the mild InC to promote the tandem carbonyl allylation and Prins cyclization reaction to give symmetrical tetrahydropyran rings [199]. Ally lie tin compounds were used for the allylation of aldehydes, and the resulting allylic alcohols proceeded to react with excess aldehydes to form either... 

Prins cyclizations, which proceed by intramolecular addition of alkenes to oxocarbenium ions, provide a simple, efficient method for the stereoselective synthesis of carbocycles and cyclic ethers [77]. Halosilanes and (la) have been used for Prins cyclizations not only as Lewis acids but also as heteroatom nucleophiles. For instance, in the presence of MesSil or MesSiBr, and lutidine, mixed acetals (26) are efficiently cyclized to 4-halotetrahydropyrans (27) with high diastereoselectivity [78]. The halide is introduced into the axial site of the C(4) position. The proposed mechanism for the MesSiBr-promoted reaction involves the initial formation of a-bromoethers (28) from (26). Solvolysis of (28) provides the intimate ion pair (29). Cyclization to the chair transition structure (30) and proximal addition of the bromide produces the observed axial adduct (27). The role of lutidine is to suppress a less selective HBr-promoted cyclization (Scheme 9.23). Acetals bearing an alkyne or allene moiety also undergo the halosilane-promoted cyclization to form haloalkenes [79, 80]. 

---