2- alkylidene-l,3-dithianes

Optically active 2-alkylidene-l,3-dithiane 1,3-dioxides have been prepared as chiral Michael-type acceptors. It was shown that these compounds react under nucleophilic epoxidation conditions to give diastereoselectively the epoxides. Other heteroatom nucleophiles reacted as well <1998JOC7128, 1999PS(153/4)337>. It was further demonstrated that enolates were also effective nucleophiles for the stereoselective addition to 2-alkylidene-l,3-dithiane 1,3-dioxides (Scheme 48) <20050L4013>. 

An intramolecular cycloaddition occurred, when 2-alkylidene-l,3-dithianes having a hydroxy group at an appropriate distant position (3- or 4-atoms) were treated with trifluoromethyl iodide in the presence of SO2. A radical mechanism with 2-alkyl-2-iodo-l,3-dithianes as intermediates is suggested (Equation 37) <1997JOC9107>. 

Radical cations of 2-alkylidene-l,3-dithianes can be generated electrochemically by anodic oxidation using a reticulated vitreous carbon (RVC) anode <2002TL7159>. These intermediates readily react with nucleophiles at C-1. Upon removal of the second electron, the sulfur-stabilized cations were trapped by nucleophilic solvents, such as MeOH, to furnish the final cycloaddition products. Hydroxy groups <20010L1729> and secondary amides <2005OL3553> were employed as O-nucleophiles and enol ethers as C-nucleophiles (Scheme 50) <2002JA10101>. 

The use of chiral 2-alkylidene-l,3-dithiane 1,3-dioxides in asymmetric cycloaddition reactions has been demonstrated. A highly enantioselective synthesis of (—)-cispentacin by an intramolecular 1,3-dipolar cycloaddition was reported (Scheme 52) <20020L1227, 20030BC684>. 

Alkylidene-l,3-dithianes, acting as acyl synthons, can be prepared by the HWE reaction of 2-phosphorylated 1,3-dithianes with aldehydes (Equation 53) <1996SL875, 1997BSE891, 1998TL5425, 2002JOC1746>. 

TL4061>, of 2-alkylidene-l,3-dithiane 1-oxides as Michael acceptors (cf. Scheme 48, Section... 

Ketene dithioketals,1 The reagent undergoes Wittig-Horner reactions with aldehydes or ketones to give 2-alkylidene-l,3-dithianes in 80-100% yield. The corresponding P=S analog reacts with aldehydes in only moderate yield (25-65% yield). 

Schuster, T., and Evans, S.A., Jr., Hetero-Diels-Alder reactions of dimethyl l-oxo-( , )-2,4-hexadi-enephosphonate with 2-alkylidene-l,3-dithianes, Phosphorus, Sulfur Silicon Relat. Elem., 103, 259, 1995. 

The addition of enolates derived from ketones and esters to 2-alkylidene-l,3-dithiane... 

Alkylidene-l,3-dithiane 5-oxides are receptive to addition of boronic acids. ... 

Alkyliden-l, 3-dithiane II, hergestellt z.B. aus dem Phosphoran I mit Aldehyden502, wer-den in Gegenwart von Quecksilber(II)-Salzen zu Carbonsauren hydrolysiert503 ... 

Alkylulen ltJ-ditUanesj These useful ketene thioacetals can be prepared as formulated in equation (I). The preparation involves N-tosylsulfilimines, which are cleaved readily to 2-alkylidene-l,3-dithianes. 

Lithio-2-trimethylsilyl-l,3-dithiane is the most widely utilized reagent for the conversion of ketones and aldehydes to the corresponding ketene dithioacetals (Scheme 2.49) [126-128]. It is used for the synthesis of functionalized 2-alkylidene-1,3-dithianes 79 [129-135]. The 2-alkylidene-l,3-dithianes 79 thus synthesized are useful synthetic intermediates, which are conveniently accessible by means of Peterson reactions, and they can be transformed into various compounds [136, 137]. For example, compounds 79 are converted to the corresponding carboxylic acids, aldehydes, and enones by hydrolysis, hydrogenation followed by hydrolysis, and deprotonation followed by alkylation and hydrolysis, respectively (Scheme 2.49) [138-140]. 

Scheme 2.49. Synthesis of 2-alkylidene-l, 3-dithianes by the Peterson reaction.

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