Intramolecular Prins-type reaction

The synthesis of different substituted finans by cyclization of 4-pentynones using potassium tert-butoxide in DMF was reported <96TL3387>. Dihydrofuran 32 can be prepared by a destannylative acylation of l-[(2-methoxyethoxy)methoxy]-2-(phenylsulfonyl)-2-(tributylstannyl)-cyclopropane. Treatment of 32 with BFj-EtjO yields 3-acyUurans via an intramolecular Prins-type reaction of the resulting oxonium ion intermediate <96TL4585>. 

Cho et al. reported a indium trichloride-catalyzed intramolecular Prins-type reactions of compounds having both the functionalities of homoallyl alcohol and acetal moiety. The intramolecular Prins cycliza-tions were performed using indium trichloride in chloroform or 25%... 

Allylsilanes react with two molar equivalents of aldehydes in the presence of AICI3 or AlBrs catalyst to give 4-halotetrahydropyrans through conventional allylation followed by formation of oxonium species and their intramolecular Prins-type reaction... 

Cyclocondensation. A Prins-type reaction between aldehydes and 3-butenol leads to 4-tetrahydropyranols and ethers. Sc(OTf)i assists epoxide opening by an intramolecular attack of an enolate, resulting in the formation of three-, four-, and five-membered rings. ... 

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... 

The putative metal cationic intermediates can be interconnected into a series of cationic processes such as Friedel-Crafts, pinacol, Nazarov, Prins-type reactions. In Scheme 7.28, the gold-catalyzed cycloisomerization of 1,6-enynes is followed by an intramolecular nucleophilic attack by the proximate carbonyl to generate the tetrahydro-pyranyl cation, which participate in a Prins-type reaction to produce tricyclic compounds resembling the core structures of several terpenoids [52]. 

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. 

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... 

The mechanism for this reaction is analogous to its intramolecular version [Ref. 223 in Chap. 1], and starts with the formation of the cyclopropyl carbene I-ll (Scheme 2.6). Direct attack of the aldehyde to the cyclopropyl gold(I) intermediate 1-11 leads to the oxonium cation 1-20, which suffers Prins-type cycUzation to give tetrahydropyranyl cation 1-21. Intermediate 1-21 can undergo metalation to yield bicycle 1-16 or can evolve by a fragmentation reaction to form 1,3-diene 1-17. 

Catalysis of Intramolecular Sakurai Reactions. Et AICI2 has been extensively used as a catalyst for intramolecular Sakurai additions. Enones (eqs 12 and 13) have been most extensively explored. Different products are often obtained with fluoride or Lewis acid catalysis. EtAlCl2 is the Lewis acid used most often although TiCLi and BF3 have also been used. EtAlCl2 also catalyzes intramolecular Sakurai reactions with ketones and other electrophiles. The cyclization of electrophilic centers onto alkylstannanes and Prins-type additions to vinylsilanes are also catalyzed by EtAlCl2. 

Recently, Barluenga and Fananas reported tandem intramolecular hydroaUc-oxylation/hydroarylation and hydroaUcoxylation/Prins-Type annulation reactions. In the first communication, they described the cycloisomerization of 5-alkynols with several gold, platinum, and silver catalysts and the application to the synthesis of enantiopure benzo fused cyclic ethers from the chiral pool [150] (Scheme 87). 

Moreover, in a subsequent paper, a reaction based on a gold- or platinum-catalyzed tandem process that involves an intramolecular hydroalkoxylation of a triple bond followed by a Prins-type cyclization has been reported for the synthesis of [3.3.1]bicyclic compounds starting from easily available alkynol derivatives [152] (Scheme 89). 

Moreover, spiroketals are produced from tandem hydroalkoxylation of 4-alkynols (Scheme 15) [130]. Starting from diynediols, bis-spiroketals are obtained using Au (I) as catalysts [131]. Furthermore, Barluenga et al. reported the formation of spirocychc compounds in a tandem alkyne hydroalkoxylation [4 -1- 2] cycloaddition reaction [132, 133], together with a tandem intramolecular hydroalkoxylation of a triple bond followed by a Prins-type cyclization [129]. 

This reaction was first reported by Kriewitz in 1899. It is the condensation between formaldehyde and olefins to form unsaturated primary alcohols under conditions of high temperature (e.g., 170°C). In this reaction, water or acetic acid is usually applied as the solvent, and an acid is added as a catalyst, such as sulfuric acid. Under these conditions, formaldehyde and some ketones function as an enophile. The double bond in the new unsaturated alcohol is adjacent to its original position in the starting olefin," with the exception of camphene, which gives an allylic alcohol. For the acylic olefins in this condensation, the isobutylene-type olefins are much more reactive than the monosubstituted olefins or 1,2-disubstituted olefins. The further extension of this reaction involving an intramolecular condensation of an a-alkoxycarbenium ion with a double bond to form a hydropyran derivative is known as the Prins-Kriewitz cyclization. ... 

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