Annulation with Allylic Silanes

Nair and coworkers reported that the choice of Lewis acid is crucial in the addition of allyltriisopropylsilane onto p-quinoneimines (37) (Equation 23) and p-benzoquinone monoimine (38) (Equation 24) [28]. When BF3 OEt2 is used for this reaction, the products obtained are indane derivatives. 

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Dipolar cycloaddition of alkenes with carbonyl ylides generated in situ is a versatile method for tetrahydrofuran synthesis. The synthetic potential of such transformations has been reviewed <2005JOM(690)5533, 2003BMI6-253>. In addition, the stereoselective [3 + 2] annulation of allyl silanes has become a reliable protocol for the synthesis of tetrahydrofurans as demonstrated in several total syntheses . Such a [3 + 2] annulation, for example, affords the tetrahydrofuran product 11 as a single stereoisomer (Scheme 15) <2002OL2945>. Lanthanide salts serve as efficient Lewis acid catalysts in similar [3 + 2] cycloaddition reactions . 

In the presence of FeCl3, l-siloxymethyl-l,4-epaxy-l,4-dihydronaphthaIe nes were converted into ortfot-naphthoquinone methides. The in situ nerated oft/io-naphthoquinone methides reacted with allyl silanes via regio- and stere-oseleaive annulations to afford dihydronaphthopyran derivatives (13AGE1515). 

Substituted (5R,6A,)-6-(dimethyl(phenyl)silyl)-2-phenyldihydropyrazolo[l,2- ][l,2,4]triazole-l,3(2//,5//)-dione 716, synthesized via the [3+2] annulation of a-substituted allylic silanes 715 with PTAD, were oxidized to the corresponding hydroxy substituted urazoles 717. This work shows that allylsilanes with a single substituent at the allylic carbon undergo exclusive stereoselective [3+2] annulation (Scheme 114) <2007TL6671>. 

The class of 3-silyl-substituted reagents provides, upon addition with aldehydes, allylic silanes that offer many options for further derivatization. Oxidative processes are described in previous sections (see the sections on Preparation of 1,2-Diols and 1,4-Diols). If the appropriate silicon substituents are chosen, formal [3+2] cycloadditions with aldehydes can be promoted under Lewis acid catalysis. For example, the mismatched addition of the Z-3-propyl-3-benzhydryldimethyl allylsilane 183 to an a-benzyloxy aldehyde proceeds with low diastereofacial selectivity in favor of product 184 however, after protection of the secondary alcohol, an efficient [3+2] annulation provides the polysubsubstituted furan 185 in good yield and acceptable stereoselectivity (Scheme 24). ° The latter is brought forward to a tricyclic unit found in the antitumor natural product angelmicin B. 

The unusual 6-azabicyclo[3.2.1]oct-3-ene core of the alkaloid (+)-peduncularine was assembled using the [3+2] annulation of an allylic silane with chlorosulfonyl isocyanate by K.A. Woerpel and co-workers. In the endgame of the total synthesis, the bicyclic aldehyde was masked as the acetal, and an efficient Fischer indole synthesis was performed using phenylhydrazine hydrochloride along with 4% H2SO4. Several subsequent steps led to the natural product. 

The one-pot three-component reaction of 2-hydroxynaphthoquinone, aromatic aldehydes and ethyl 4,4,4-trifluoro-3-oxobutanoate in the presence of ammonium acetate and acetic acid affords a series of 2-trifluoromethylated benzol ]chroman derivatives (13S2193).Various allyl silanes are suitable for an annulation reaction with 1-naphthoquinone-2-methide, formed through an FeCl3-mediated transformation of l,4-epoxy-l,4-dihydronaphthalenes, to prepare benzo[/z]chromans, in moderate to good yields (Scheme 36) (13AGE1515). 

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