Dithianes carbonyl protecting group

Dithioketals, especially the cyclic dithiolanes and dithianes, are also useful carbonyl-protecting groups. These can be formed from the corresponding dithiols by Lewis acid-catalyzed reactions. The catalysts that are used include BF3, Mg(03SCF3)2, Zn(03SCF3)2, and LaCl3.100 S-Trimethylsilyl ethers of thiols and dithiols also react with ketones to form dithioketals.101... 

The next carbonyl protecting group stems from an attempt to retain the carba-nion-stabilising properties of the dithiane 45-acetal whilst increasing its hydro-... 

Similarly, in another example, alkylation of 111 with diepoxide (—)-115 (1 equiv.) in the presence of HMPA (1.3 equiv.) furnished diol (+)-117. Protection of (+)-117 to form the acetonide, removal of the silyl protecting groups (TBAF), and hydrolysis of the dithiane with Hg(Cl04)2 provided the diketone (+)-118. Hydroxy-directed syn-reduction of both carbonyl groups with NaBI U in the presence of Et2BOMe, and triacetonide formation, followed by hydrogenolysis and monosilylation, afforded the desired Schreiber subtarget (+)-119, which was employed in the synthesis of (+)-mycoticins A and B (Scheme 8.31) [56b]. 

Carbon-13 shifts of some 2-substituted 1,3-dithianes as protected carbonyl derivatives and nucleophilic carbonyl equivalents are collected in Table 4.63 [413], Methyl group increments for shift predictions according to eq. (4.1) are available, also taking contributions of the twist in addition to the chair conformation into account [413]. 

McHale, W.A. and Kutateladze, A.G. (1998) An efficient photo-SET-induced cleavage of dithiane-carbonyl adducts and its relevance to the development of photoremovable protecting groups for ketones and aldehydes. Journal of Organic Chemistry, 63, 9924—9931. 

RCeCl2. The reagents, prepared in situ from RLi + CeCK, are more reactive than RLi in 1,2-addition to carbonyl groups. An example is the reaction with 2-alkoxypropionoyl-l,3-dithianes (1), prepared from methyl (S)-lactate.2 The dias-tereoselectivity obtains even when the protecting group is incapable of chelation. 

Protection and Deprotection of Amino-acids.— The 1,3-dithian-2-ylmethoxy-carbonyl (Dmoc) group in an amino-protecting group which is stable to bases and to trifluoroacetic acid it is removed by oxidation of the sulphur atoms and... 

Dithianes are known as protecting groups for safety-catch linkers [66] but they can also be used as Unker systems for the synthesis of either carbonyl-containing or aliphatic compounds. Bertini et al. [280] synthesized the first... 

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. 

An unexpected reactivity in the functionalization of 2-acyl-l,3-dithianes has been reported by Mioskowski and co-workers. They found that 2-acyl-l,3-dithianes with no further heteroatom at the acyl side chain react with aldehydes to give 2-acyl-2-hydroxyalkyl-l,3-dithianes, whereas a silyl-protected hydroxy group in the side chain of the 2-acyl-l,3-dithiane led to formation of the aldol product at the opposite site of the carbonyl group. Acyl chlorides always react with 2-acyl-l,3-dithianes to give the enol esters (Scheme 81) <2003TL213>. 

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