Dithianes hydrolysis

In a synthesis of Epothilones A and B, Danishefsky and co-workers used a triphenylsilyl ether to protect a hindered secondary alcohol in a long sequence that included DDQ deprotection of a benzyl ether, dithiane hydrolysis, dimethyl acetal hydrolysis with p-toluenesulfonic acid in dioxane-H20 (5 1) at 50 °CT... 

Sainsbury and co-workers (121) have synthesized several ellipticine dimers tethered through the C-5 methyl group (333) (Scheme 53) or the C-9 position (334). The 9-methoxy derivative of 333 was also prepared. The nitrile 329 was available from the Sainsbury ellipticine synthesis (122) and was transformed into the alkaloid 17-oxoellipticine (148). A clever maneuver was to add nitric acid to protonate the pyridine nitrogen of 330. This precluded A-oxide formation during dithiane hydrolysis. Reductive amination in two steps afforded the amine 332. Coupling with adipic acid gave the target bisellipticine 333. 

Additionally, Schaumann demonstrated that this approach can be used in the S3Uithesis of pyrrolidine- and piperidine-2,3-diones. Treatment of bromoalkyl isocyanates with 2-lithio-2-TMS-l,3-dithiane provides, after Brook rearrangement and nucleophilic displacement of bromide, the corresponding lactams with moderate to good yields. Protection of the nitrogen followed by a two-step dithiane hydrolysis affords pyrrolidine- and piperidine-2,3-diones (eq 25) ... 

Lithium l,3-dithian-2-ides (p. 6, 8) may be alkylated with alkyl bromides or iodides. Steric hindrance is usually of little importance and the resulting ketone can be easily liberated by hydrolysis (D. Seebach, 1969). 

Hydrolysis of tfie resulting dithiane yields a ketone. 

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. 

Acyclic monothio acetals and ketals can be prepared directly from a carbonyl compound or by transketalization, a reaction that does not involve a free carbonyl group, from a 1,3-dithiane or 1,3-dithiolane. They are cleaved by acidic hydrolysis or Hg(II) salts. 

The (V-methyldihydrodithiazine 125 has also been used as an effective formyl anion equivalent for reaction with alkyl halides, aldehydes, and ketones (77JOC393). In this case there is exclusive alkylation between the two sulfur atoms, and hydrolysis to give the aldehyde products is considerably easier than for dithianes. However, attempts to achieve a second alkylation at C2 were unsuccessful, thus ruling out the use of this system as an acyl anion equivalent for synthesis of ketones. Despite this limitation, the compound has found some use in synthesis (82TL4995). 

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]. 

Other carbanionic groups, such as acetylide ions, and ions derived from a-methylpyridines have also been used as nucleophiles. A particularly useful nucleophile is the methylsulfinyl carbanion (CH3SOCHJ), the conjugate base of DMSO, since the P-keto sulfoxide produced can easily be reduced to a methyl ketone (p. 549). The methylsulfonyl carbanion (CH3SO2CH2 ), the conjugate base of dimethyl sulfone, behaves similarly, and the product can be similarly reduced. Certain carboxylic esters, acyl halides, and DMF acylate 1,3-dithianes (see 10-10. )2008 Qxj(jatjye hydrolysis with NBS or NCS, a-keto aldehydes or a-... 

Alkylation and hydrolysis of imines alkylation of aldehydes 10-107 Alkylation and hydrolysis of dithianes 10-108 Alkylation and hydrolysis of oxazines and similar compounds 10-109 Reaction of diazo aldehydes with boranes... 

Alkylation and hydrolysis of dithianes and similar compounds 10-108 Alkylation and hydrolysis of oxazines... 

Some cyclic thioacetals have an A-SE2 hydrolysis mechanism,206 as do some 2-aryl-2-methyl-l,3-dithianes, except for the 4-NO2 derivative, which looks more A2-like.207 In 10 vol% dioxane/aqueous HC104 mixtures, reactive 2-aryl-2-phenyl-l,3-dithianes are believed to have an A-SE2 hydrolysis mechanism, whereas the least reactive ones have an A2 mechanism.130 Isothiocyanates are believed to hydrolyze by a mechanism that involves simultaneous proton transfer to nitrogen and attack of water at carbon in a cyclic transition... 

Chlorinated ketals 221 were lithiated using a catalytic amount (8%) of naphthalene in THE at —78°C to generate the corresponding masked lithium 5-enolates 222, which upon treatment with different electrophiles in THE at temperatures ranging between —78 and 20 °C, and final hydrolysis with water, afforded protected functionalized ketals 223 (Scheme 76). The application of this methodology to the chlorinated dithiane 224, under the same reaction conditions, gave the intermediate 225 and finally products 226 (Scheme 76) -... 

The y-cthylthiosulfoxides can be converted to the corresponding carbonyl compounds by hydrolysis catalyzed by mercuric ion. In both the dithiane and alkylthiomethylsulfoxide systems, an umpolung is achieved on the basis of the carbanion-stabilizing ability of the sulfur substituents. 

The efficient resolution of /ra r-4,5-dihydroxy-l,2-dithiane into the two enantiomers in large quantities has been reported by the reaction of the racemic mixture with the amino acid iV-/-butoxycarbonyl-(5)-phenylalanine <1997TL7657>. By fractional crystallization, the (43, 53 )- and (4/J,5iJ)-esters were separated followed by hydrolysis, which provided the desired enantiomeric diols in excellent yield and >99% ee. These reactive diols provide isomerically pure analogs with interesting selectivity and therapeutic potential for example, 4,5-dihydroxy-l,2-dithiane derivatives have been reported to inhibit the replication of HIV-1 and HIV-2 (human immunodeficiency viruses). 

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