For 1,3-dithiolane

For 1,3-dithiolanes the ring is flexible and only small energy differences are observed between the diastereoisomeric 2,4-dialkyl derivatives. The 1,3-oxathiolane ring is less mobile and pseudoaxial 2- or 5-alkyl groups possess conformational energy differences (cf. 113 114) see also the discussion of conformational behavior in Section 4.01.4.3. 

Rudrawar S, Besra RC, Chakraborti AK (2006) Perchloric acid adsorbed on silica gel (HC-lO -SiOj) as an extremely efficient and reusable catalyst for 1,3-dithiolane/dithiane formation. Synthesis 16 2767-2771... 

A carbonyl group can be protected as a sulfur derivative—for example, a dithio acetal or ketal, 1,3-dithiane, or 1,3-dithiolane—by reaction of the carbonyl compound in the presence of an acid catalyst with a thiol or dithiol. The derivatives are in general cleaved by reaction with Hg(II) salts or oxidation acidic hydrolysis is unsatisfactory. The acyclic derivatives are formed and hydrolyzed much more readily than their cyclic counterparts. Representative examples of formation and cleavage are shown below. 

For (n = 2, 3) A chlorobenzotriazole, CH2CI2, —80° NaOH, 50% yield.1,3-Dithianes and 1,3-dithiolanes, used in this example to protect C3-keto steroids, were not cleaved by HgCl2-CdC03. 

Although deprotonation of simple 1,3-dithiolanes at the 2 position is usually accompanied by cycloreversion to the alkene and dithiocarboxylate, this does not occur for the 2-ethoxycarbonyl compound 55. The anion of this is readily generated with LDA and undergoes conjugate addition to a,(3-unsaturated ketones, esters, and lactones to give, after deprotection, the a,8-diketoester products 56 (73TL2599). In this transformation 55 therefore acts as an equivalent of Et02C-C(0) . 

The modified Sharpless reagent was also successfully applied288 for the asymmetric oxidation of a series of 1,3-dithiolanes 248 to their S-monooxides 249 (equation 134). It was observed that the optical induction on sulphur (e.e. from 68 to 83%) is not significantly affected by the substituents R1 and R2. Asymmetric oxidation of a few aryl methyl sulphides by organic hydroperoxides in the presence of a catalytic amount of the optically active Schiff base-oxovanadium(IV) complexes gave the corresponding sulphoxides with e.e. lower than 40%289. 

The substitution of a heteroatom for an a-sulfoxy methylene group substantially increases the preference for an axial orientation of the sulfoxide oxygen320, despite the smaller space requirement of the sulfur with its lone pairs, compared to that of a methylene group321, at least in the case of 1,3-dithiolane oxides. The substituting heteroatom, therefore, should decrease the conformation stability (i.e. lower the barrier to chair-chair interconversion). 

Herein is described a much simpler dehydrohalogenation alternative that had been earlier applied successfully to the preparation of ketene acetals6-7 and 2-alkylidene-1,3-dithianes.8 This route appears not to have been examined for preparing the title compound because of an early report that 2-lithio-1,3-dithiolanes undergo ready fragmentative elimination to form ethylene and dithlocarbonate unlike... 

Secondary alkyl selenides are reduced by (TMS)3SiH, as expected in view of the affinity of silyl radicals for selenium-containing substrates (Table 4.3) [40]. Reaction (4.23) shows the phenylseleno group removal from the 2 position of nucleoside [50]. Similarly to 1,3-dithiolanes and 1,3-dithianes, five- and six-membered cyclic selenoacetals can be monoreduced to the corresponding selenides in the presence of (TMS)3SiH [51]. The silicon hydride preferentially approached from the less hindered equatorial position to give transicis ratios of 30/70 and 25/75 for the five-membered (Reaction 4.24) and six-membered cyclic selenoacetals, respectively. 

BnPh3PHS05 was used for deprotection of oximes and semicarbazones to their parent carbonyl compounds under microwave irradiation. Similarly, dethioacetalization of 1,3-dithiolanes and 1,3-dithianes by BnPhsPHSOs in aprotic solvents was also reported . In both these reactions a catalytic amount of bismuth chloride was necessary. 

Scheme 1.64). The Ag(I)-mediated cyclization afforded dipole 306 for 1,3-dipolar cycloaddition with methyl vinyl ketone to yield adducts 307 and the C(2) epimer as a 1 1 mixture (48%). Hydrogenolytic N—O cleavage and simultaneous intramolecular reductive amination of the pendant ketone of the former dipolarophile afforded a mixture of alcohol 308 and the C(6) epimer. Oxidation to a single ketone was followed by carbonyl removal by conversion to the dithiolane and desulfurization with Raney nickel to afford the target compound 305 (299). By this methodology, a seven-membered nitrone (309) was prepared for a dipolar cycloaddition reaction with Al-methyl maleimide or styrene (301). 

Historically, the first reactions involving thiocarbonyl ylides involve the preparation of thiiranes and 1,3-dithiolanes from diazomethane and thiocarbonyl compounds reported early in the last century by Staudinger and co-workers (12,13). Similar reactions have been smdied by Schonberg and co-workers (14—16) during the 1960s, but neither was the reaction mechanism understood nor have thiocarbonyl ylides been recognized as key intermediates. [For some remarks to this subject see (8) and (10) in (17).]... 

Based on a series of kinetic studies, Huisgen et al. (91-93) established that thiocarbonyl compounds, especially aromatic thioketones, function as very active dipolarophiles (superdipolarophiles) toward thiocarbonyl ylides. In fact, the trapping reaction of thiocarbonyl ylides with thiocarbonyl compounds represents an excellent method for the preparation of 1,3-dithiolanes. 

Other C=S compounds that have proven to be useful for the preparation of 1,3-dithiolanes are dithioesters, (161-163) trithiocarbonates, (161) and 1,3-thiazole-5-(4//)-thiones (164). 

V(V), Mo(VI) and Ti(IV) derivatives proved to be both effective and selective in the oxidation of sulfides with alkyl hydroperoxides324. As for H2O2, vanadium is at least twofold more efficient than molybdenum in oxidation and much more selective, as proved by the data obtained with the cyclic disulfide 2-aryl-1,3-dithiolane 80. A large predominance of the tra .y-S-oxide 81 over the c/.v-dcri vati ve 82 is obtained for all the systems investigated (equation 46). However, the diastereoselectivity exhibited by V(V)/TBHP is remarkable315-325. 

Fluorination of 1,3-dithiolanes in aqueous acetonitrile offers novel methodology for the deprotection of thiolanes to the parent ketones (Fig. 91) [162]. 

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