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Homoallylic prins cyclization

Aza-Prins Cyclization of Epoxides with Homoallylic Amines. 234... [Pg.230]

Fig. 3 Aza-Prins cyclization of epoxides with N-protected homoallylic amines... Fig. 3 Aza-Prins cyclization of epoxides with N-protected homoallylic amines...
Prins cyclization reaction of scalemic homoallylic alcohols (26) with aldehydes (R CIIO), carried out in the presence of an acid catalyst (HX), affords tetrasub-stituted tetrahydropyrans (27) (99% ee) with high stereoselectively in good yields... [Pg.324]

Anhydrous iron(III) halides catalyse coupling of alkynes and aldehydes.211 Simple terminal alkynes, R CH, react with aldehydes, R2CHO, to give ( ,Z)-1,5-dihalo-1,4-dienes (55). In contrast, non-terminal arylalkynes give ( ,)-o, /3-unsaturated ketones. The catalysts also promote standard Prins cyclization of homoallylic alcohols. Studies of intermediates and of alkyne hydration - together with calculations - all support FeX3 complex formation with alkyne as the activating step. [Pg.24]

A stereocontrolled synthesis of 2,4,5-trisubstituted tetrahydropyrans 331 can be achieved via a Lewis-acid-catalyzed intramolecular Prins cyclization of homoallylic acetals 332. Incorporation of a variety of substituents at C-4 of the resulting tetrahydropyrans is possible by simple variation of the reaction conditions (Equation 141, Table 13) <2001CC835>. [Pg.496]

The intermolecular Prins cyclization between ketones and homoallylic alcohols can be catalyzed by mercuric triflate <1999TL1153>. TMSNTf2 is an effective catalyst for the reaction between aldehydes and homoallylic alcohols <2002AGE161>. [Pg.501]

Dibromotetrahydropymns. Prins cyclization involving RCHO and homoallylic alcohols bearing a bromine atom at the far end of the double bond, as induced by InBr3 and terminated by Me3SiBr, is stereoselective. [Pg.254]

Iron nitrate supported on bentonite induces the sequential oxidation of benzyl alcohols, and Prins cyclization of the aldehydes formed, with homoallylic alcohols and trimethylsilyl chloride to provide 2-aryl-4-chlo-rotetrahydro-2Ef-pyrans (Scheme 15) (13SL1781). [Pg.472]

A novel and rapid synthesis of tetrahydropyran derivatives through the Prins cyclization reaction of homoallyl alcohols with aldehydes in the presence of bismuth trichloride was described. The reaction proceeded rapidly under microwave irradiation in solvent-free conditions. The conversions were clean and the products were obtained in excellent yield with high diastereoselectivity (Equation 24) [43a]. [Pg.34]

Shortly after the synthesis by Floreancig group. Keck and co-workers completed the synthesis of (+)-dactylolide. Retorosynthetically, Keck s approach to dac-tylolide is similar to those of previous syntheses, in that the major disconnection at C(2)-C(3) at the late stage macrocyclization is to be achieved through Horner-Wadsworth-Emmons olefination and the Prins cyclization via an allyl silane addition to an aldehyde to build up the 2,6-cw-tetrahydropyran unit (Scheme 2.57). However, preparation of the homoallyl alcohol 2.274 fragment via Keck s asymmetric aUylation is unique as illustrated in Scheme 2.58. [Pg.122]

Stemming from the studies on the oxonia [3,3]-sigmatropic alkyl migration of branched homoallylic alcohols by Loh et al. (see Section 8.15, Figure 8.132), the same group had also found that In(OTf)3 catalyzes the formation of tetrahydrofu-rans and tetrahydropyrans from homoallylic alcohols in the presence of aldehydes, developing their initial observations to generate crossed Prins cyclization products... [Pg.429]

Dobbs et al carried out the Prins cyclization between homoallylic alcohols, thiols or amines and aldehydes, mediated by InCb [209]. This procedure allowed for the generation of unsaturated heterocycles using mild conditions. Interestingly, for homoallylic amines, the product possesses an anti relative stereochemistry instead of the typical syn observed in most Prins cyclizations (Figure 8.94). [Pg.431]

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]. [Pg.64]

Indium Lewis acids have garnered attention due to their mild reactivity and air and water stability. Both Li et al. and Chan and Loh have shown that In(III) complexes are suitable Lewis acids for Prins cyclizations [81, 82]. These reports prompted Loh and coworkers to embark on a synthesis of (+)-SCH 351488 that utilized this strategy (Scheme 40) [83]. Condensation of homoallylic alcohol 147 and aldehyde 148 in the presence of indium ttibromide and TMSBr gave 4-bromo THP 149 in 65 % overall yield as an inconsequential mixture of diastereomers (2,A-cisP,4 trans = 75 25). Complete retention of the homoallylic alcohol stereochemistry is responsible for the key 2,6-cis relationship in the product. Initial attempts to apply these same conditions to the B ring resulted in acetonide deprotection and no THP formation. Subsequent optimization revealed that indium triflate and TMSCl were competent additives to effect cyclization. Careful temperature control was required to suppress an undesired Prins side reaction. The combination of homoallylic alcohol 150 and aldehyde 151 in the presence of the appropriate Lewis acids at 78 °C, followed by warming to 0 °C for 4 h, led to the desired monomer precursor 152 in 42 % yield. [Pg.67]

Scheme 1). Introduction of a jt bond into the molecular structure of 1 furnishes homoallylic amine 2 and satisfies the structural prerequisite for an aza-Prins transform.4 Thus, disconnection of the bond between C-2 and C-3 affords intermediate 3 as a viable precursor. In the forward sense, a cation ji-type cyclization, or aza-Prins reaction, could achieve the formation of the C2-C3 bond and complete the assembly of the complex pentacyclic skeleton of the target molecule (1). Reduction of the residual n bond in 2, hydro-genolysis of the benzyl ether, and adjustment of the oxidation state at the side-chain terminus would then complete the synthesis of 1. [Pg.466]

A fourfold anionic sequence which is not initiated by a Michael but an aldol reaction has been reported by the group of Suginome and Ito (Scheme 2.129) [295]. In this approach, the borylallylsilane 2-573 reacts selectively in the presence of TiCl4 with two different aldehydes which are added sequentially to the reaction mixture. First, a Lewis acid-mediated allylation of the aldehyde with 2-573 takes place to form a homoallylic alcohol which reacts with the second aldehyde under formation of the oxenium ion 2-574. The sequence is terminated by a Prins-type cyclization of 2-574 and an intramolecular Friedel-Crafts alkylation of the intermediate 2-575 with formation of the fraws-1,2-be rizoxadeca lines 2-576 as single diastereomers. [Pg.137]

Tetrahydropyrans hydroxylated at the 4-position have good synthetic value [113]. Although many synthetic methods have been reported [17-23,114,115], the search for potential alternate approaches and the development of eco-friendly and high-yielding reactions resulted in the development of a method that poses less problems for the environment. Synthesis of tetrahydropyranol derivatives can be achieved through the Prins-type cyclization reaction of homoallylic alcohols with aldehydes using bismuth triflate as catalyst in [bmim]PF6 solvent system [108] (Fig. 22). [Pg.247]

While several mechanisms for the Prins reaction have been proposed,7,10 there is general agreement that the mechanism shown below is reasonable.7 Attack of olefin 1 on protonated aldehyde 10 gives carbocation 11. This carbocation can then be trapped with another molecule of aldehyde, generating adduct 12. Cyclization of 12 affords dioxane 3. Carbocation 11 can also be captured with water giving diol 4. Finally, elimination of a proton from carbocation 11 affords homoallylic alcohol 5. It is possible that the formation of 5 could also proceed via an ene-type mechanism.8... [Pg.653]

Formaldehyde can be coupled to an alkene in the presence of an acid to give a diol (152) or a 1,3-dioxane derivative (154) in what is known as the Prins reaction. l Allylic alcohols such as 153 can also be produced in this reaction. Camphene (155) reacted with formaldehyde and acid to give a 1 1 mixture of allylic alcohol 156 and the acetate 157, in 94% yield. Scandium tiiflate has been used to prepare tetrahydropyran-4-ol derivatives from aldehydes and homoallylic alcohols via a Prins-type cyclization. 3... [Pg.1079]

Cyclizations. In addition to the use of SEM-Cl as a means of one-carbon homologation, it has also been employed in the generation of acetals capable of undergoing a variety of cy-clization events. Alkylation of allylic alcohols with SEM-Cl permits haloacetalization to afford di- and trisubstituted 1,2-dioxanes (eq 28). Alternatively, homoallylic alcohols permit Prins-type cyclizations (eq 29) to furnish 2,4,5-trisubstituted tetrahydro-pyrans. Transition metals can also be employed to promote the formation of substituted benzofurans (eq 30). ... [Pg.632]

Cyclization via Prins-Type Reactions The intramolecular Prins reaction has been recognized as a powerful method to assemble six- and seven-membered rings from homoallylic alcohols and simple aldehydes under acid catalysis [28]. However, the competing 2-oxo-Cope rearrangement prior to cyclization could lead to a product with partial or total racemization when enantiomerieally enriched secondary alcohols are used [29]. Increasing the nueleophilicity of the alkene reagent... [Pg.100]

The homoallylic alcohol (90) is cyclized to s-cadinene (91) by reaction with methanesulfonyl chloride in pyridine 145). An intramolecular Prins reaction of the corresponding ketone (preisocalamendiol, 92) is effected by treatment with aqueous acetic acid, giving isocalamen-diol (93) 146). [Pg.109]


See other pages where Homoallylic prins cyclization is mentioned: [Pg.137]    [Pg.234]    [Pg.24]    [Pg.403]    [Pg.501]    [Pg.95]    [Pg.466]    [Pg.302]    [Pg.126]    [Pg.429]    [Pg.449]    [Pg.62]    [Pg.62]    [Pg.63]    [Pg.70]    [Pg.91]    [Pg.220]    [Pg.371]    [Pg.473]    [Pg.473]   
See also in sourсe #XX -- [ Pg.431 ]




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