Cyclization unsaturated acetals

Aubele et al. studied the aqueous Prins cyclization using cyclic unsaturated acetals as oxocarbenium ion progenitors and allylsilanes are used as nucleophiles. Cyclizations proceed efficiently inside Lewis acidic micelles (of cerium salt) in water. A variety of vinyl- and aryl-substituted tetrahydropyrans with excellent stereocontrol was obtained (Eq. 3.26).113... 

Alkenyloxazolidines.1 In the presence of pyridinium tosylate or BF, eth-erate, the N-protected norephedrine (1) cyclizes with the a,(3-unsaturated acetal 2 to give the 2-alkenyloxazolidine 3 as the major product. Cuprates add to 3 from the si face with high selectivity to give adducts (4), which are readily converted to (S)-3-alkylsuccinaldehydes in high enantiomeric purity. 

Medium-ring cyclic ethers. Unsaturated eight- and nine-membered cyclic ethers can be obtained by cyclization of unsaturated acetals with SnCl4 (2 equiv.) in CH2C12. 

Another method to construct methoxysubstituted cyclopropyl ketones is outlined in Eq. 25. Here acid chlorides arid allyl chloride are combined to give 84 which could be cyclized to the key compound 85 after addition of methanol. Standard methodology brings about preparation of y-oxoaldehydes 86 or j3,y-unsaturated acetals 8751). 

Scheme 15. Radical-chain cyclization of unsaturated acetals using polarity reversal catalysis...

Oxocanyl carbenium ions (42) are strongly indicated in the Lewis acid-catalyzed cyclization of 5-hexenylacetals. The evidence for formation of these species includes (i) the presence of aldehyde by-products (ii) the isolation of 2-alkoxyoxocanes (oxocanylacetals) (iii) intramolecular trapped products for some unsaturated acetal derivatives and (iv) the formation of 2-alkylated oxocanes when alkylaluminum complexes are used as the Lewis acid catalyst. 

Thiol-catalyzed Radical-chain Cyclization of Unsaturated Acetals and Thioacetals. When the unsaturated dioxolane 1 and a radical initiator, 2,2-di(t-butylperoxy)butane (DBPB), were heated at 125 °C in octane in the presence of tri(f-butoxy) silanethiol (TBST), the spirocyclic ketal 2 was formed cleanly and isolated in 92% yield (eq 1). When the reaction was performed in the absence of TBST, compound 2 was not detected. TBST is... 

The synthesis of chiral piperidines has been accomplished through an intramolecular cyclization reaction using chiral phosphoric acid catalysts (Scheme 3.31) [29], Treatment of protected amines bearing tethered unsaturated acetals with catalytic amounts of a chiral phosphoric acid catalyzed the formation of chiral piperidines with high selectivity. 

Unusual cyclocarbonylation of allylic acetates proceeds in the presence of acetic anhydride and an amine to afford acetates of phenol derivatives. The cinnamyl acetate derivative 408 undergoes carbonylation and Friedel-Crafts-type cyclization to form the a-naphthyl acetate 410 under severe condi-tions[263,264]. The reaction proceeds at 140-170 under 50-70 atm of CO in the presence of acetic anhydride and Et N. Addition of acetic anhydride is essential for the cyclization. The key step seems to be the Friedel-Crafts-type cyclization of an acylpalladium complex as shown by 409. When MeOH is added instead of acetic anhydride, /3,7-unsaturated esters such as 388 are... 

Finally, in a study of Lewis-acid-catalysed intramolecular attack of acetals on vinylsilanes, to produce allylically unsaturated oxacyclics, it has been found (75) that the alkene stereochemistry can control the mode of cyclization in an exo- or endocyclic sense, as shown here ... 

Additions of silylated ketene acetals to lactones such as valerolactone in the presence of triphenylmethyl perchlorate in combination with either allyltrimethylsilane 82, trimethylsilyl cyanide 18, or triethylsilane 84b, to afford substituted cyclic ethers in high yields have already been discussed in Section 4.8. Aldehydes or ketones such as cyclohexanone condense in a modified Sakurai-cyclization with the silylated homoallylic alcohol 640 in the presence of TMSOTf 20, via 641, to give unsaturated cyclic spiro ethers 642 and HMDSO 7, whereas the 0,0-diethyllactone acetal 643 gives, with 640, the spiroacetal 644 and ethoxytrimethylsilane 13b [176-181]... 

Acetalization or ketalization with silylated glycols or 1,3-propanediols and the formation of thioketals by use of silylated 1,2-ethylenedithiols and silylated 2-mer-captoethylamines have already been discussed in Sections 5.1.1 and 5.1.5. For cyclizations of ketones such as cyclohexanone or of benzaldehyde dimethyl acetal 121 with co-silyl oxyallyltrimethylsilanes 640 to form unsaturated spiro ethers 642 and substituted tetrahydrofurans such as 647, see also Section 5.1.4. (cf. also the reaction of 654 to give 655 in Section 5.2) Likewise, Sila-Pummerer cyclizations have been discussed in Chapter 8 (Schemes 8.17-8.20). 

Radical cyclization reactions have been extensively applied in synthesis. Among the first systems to be studied were unsaturated mixed acetals of bromoacetaldehyde.323... 

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