Cyclizations hexamethyldisilazide

Spirocyclic oxindole 60 was synthesized by [3,3]-sigmatropic rearrangement of the Af-phenyl-O-acylhydroxamic acid 58 (equation 19). The potassium enolate formed by treatment of 58 with potassium hexamethyldisilazide at low temperature rearranged to 59, which easily cyclized to the spirocyclic oxindole 60. Spirooxindoles were previously synthesized by Wolff and Taddei. The spirooxindole 60 was formed in 51% yield from cyclohexanecarboxylic acid after heating the preformed lithium salts of phenyl hydrazide 61 to 205-210 °C. 

An elegant cyclization-cleavage strategy has been devised for the removal of resin-bound 1,3-amino alcohol derivatives 392 as l,3-oxazin-2-ones 393 upon treatment with lithium hexamethyldisilazide (LiHMDS) (Equation 42) <20010L3177>. 

The phosphorylated bromobenzamide derivative 180 cyclizes to produce the phosphorylated isoindolinone on exposure to an excess of potassium hexamethyldisilazide (KHMDS) (Equation 116) <2000T1491>. 

Page et al. (see [298] and references therein) have shown that generally excellent stereocontrol in organic reactions can be obtained by using DITOX (1,3-dithiane-l-oxide) derivatives as chiral auxiliaries. The one-pot stereo-controlled cycloalkanone synthesis given here outlines some aspects of the chemistry worked out for efficient acylation-alkylations steps. Of note are the use of N-acyl imidazoles under mixed base (sodium hexamethyldisilazide/n-butyllithium) conditions to yield the lithium enolates of 2-acyl-l,3-dithiane-l-oxides) and the sequential alkylation-cyclization of the latter (steps (iv) and (v)). 

DIAZO TRANSFER / -Tolucncsulfonyl azide. DICHLOROVINYLATION Trichloroethylene. DIECKMANN CYCLIZATION Lithium hexamethyldisilazide. 

The /3-amino esters 387, obtained by hydrolysis of the corresponding /3-amino amides, have been cyclized in the presence of lithium hexamethyldisilazide (LHMDS)/THF to furnish the /ram-3,4-disubstituted azetidin-2-ones (Scheme 58) <2001JOC9030>. 

For the corresponding cyclization of acetylenic amides 418, Nagasaka etal.347,347a used a mixture of silver(i) triflate and lithium hexamethyldisilazide as the precatalyst. The process is probably initiated by the coordination of AgN(SiMe3)2 (generated in situ from AgOTf and LHMDS) to the triple bond, followed by nucleophilic attack of the lithium amide at the activated alkyne which affords (ZVy-alkylidene-y-butyrolactams 419 in high yields (Scheme 122). 

Ring opening of oxiranes with iodotrimethylsilane provides silylated halohydrins <84CHEC-l(7)in). An exception to this reaction mode has been reported <9iJOC4598>. Silyl enol ether-terminated oxiranes, when treated with TMS-I in the presence of hexamethyldisilazide (HMDS) at low temperatures suffer C—C cleavage and cyclization to dihydrofurans. 

Both LHMDS and KHMDS have been successfully utilized as nitrogen sources in palladium(O) catalyzed aminations of allyl chlorides (eq 28). Up to 90% conversion and 55% isolated yield were obtained for the synthesis of allyl hexamethyldisilazide and no cyclization via deprotonation or significant hydrolysis during the work-up was observed. ... 

The group of Lai and Rafii prepared a chiral imidazolium tetrafluoroborate from commercially available (lR,2R)-l,2-diamlnocyclohexane (Scheme 2.167) [61]. Palladium-catalyzed coupling of the diamine and mesityl bromide with 3 equiv of NaOtBu produced a diamine in the first step. Cyclization of this material with triethyl orthoformate in the presence of ammonium tetrafluoroborate led to the chiral BF, salt in an almost quantitative yield [62]. Finally, the chiral rhodium complex was synthesized by reacting the ligand precursor with an equimolar amount of potassium hexamethyldisilazide, followed by coordination to rhodium with [RhCl(COD)]2. 

A new and convenient synthesis of small-ring ketones is based on the anion derived from a protected cyanohydrin. The cyclization step (14) - (15) proceeds in 62% yield by employing sodium hexamethyldisilazide in refluxing benzene (Scheme 1). 

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