Triflates catalytic hydrogenation

Treatment of 8-azidomethylperhydropyrido[l,2-c]pyrimidin-l-one 157 with methyl triflate and catalytic hydrogenation of the azide group led to the formation of tricyclic guanidine derivative 158 (01JA8851). Hydroxy group of 149 was protected with methoxymethyl chloride, and the p-methoxybenzyl protecting group (PMB) was eliminated by treatment with DDQ. 

Vinyl ethers and vinyl esters are not employed extensively in organic transformations. During the synthesis of phlegmarine, however, the stereogenic center at C-10 was to be introduced stereoselectively by catalytic hydrogenation-hydrogenolysis of the vinyl triflate (Scheme 4.7). 

There have been a number of reports where alicyclic-bridged precursors underwent an IAAC reaction. Thus, the dioxolane (203b), formed from triflate (203a), cyclized in situ to a tricyclic triazoline (Scheme 63).113 Treatment of this triazoline with sodium ethoxide converted it to a diazopyrrolidine in 86% yield, which underwent smooth catalytic hydrogenation in 89% yield. The (Z)-azidoalkene (204), bridged by a (3-lactam, cyclized at 20 °C to triazoline (205).114 The triazoline (205) extruded nitrogen at 80 C providing a tricyclic aziridine. The ( )-isomer of (204) did not cyclize to a triazoline but instead produced an azirine, presumably via a nitrene intermediate. 

Step 6a Catalytic hydrogenation reduces the vinyl triflate moiety and cleaves the Cbz protecting group. 

Treatment of substrate 46 with LDA (THF, 0°C) under irradiation effected cyclization to afford the enolate 47, which is trapped by reaction with A-phenyltriflimide to give the vinyl triflate 48 in 59% yield. Catalytic hydrogenation reduced the vinyl triflate to the saturated hydrocarbon with... 

Alkenylboron compounds couple with the representative organic halides or triflates (Scheme 28). Hexaalkylbenzene was synthesized by sixfold alkenylation (55) of hexabromobenzene followed by catalytic hydrogenation of the double bonds 11521. The reaction of 1-alkenylborane with 1-bromo-l-alkyne stereose-lectively provided ( )-enyne (56) which was then converted into ( ,Z)-hexa-deca-10,12-dienal, a sex pheromone of the melonworm 11531. Due to the difficulty of purification of a geometrical mixture, the stereoselective synthesis is critical for such dienes or trienes. The PGEi derivatives (57) were synthesized... 

The enantioselective total synthesis of (-)-hemiasterlin, a marine tripeptide with cytotoxic and antimitotic activity, was achieved by E. Vedejs and co-workers. The asymmetric Strecker reaction was used to construct the key tetramethyltryptophan subunit. The aldehyde substrate was first converted to the corresponding chiral imine with (R)-2-phenylglycinol under scandium triflate catalysis. The addition of tributyltin cyanide resulted in the formation of a-amino nitriles as an 8 1 mixture of diastereomers. Subsequently the cyano group was converted to a primary amide, and the chiral auxiliary was removed under catalytic hydrogenation conditions. 

The first place in catalytic hydrogenation nowadays is taken by Rh or Ru complexes of BINAP. This ligand has axial chirality as the naphthalene rings cannot rotate past each other. These compounds were developed by Noyori, who with Knowles and Sharpless received the 2001 Nobel prize for their contributions to asymmetric synthesis. BINAP 20 is usually made from BINOL 19 and either 19 or 20 can be resolved. Rhodium complexes similar to those we have met include a molecule of cyclooctadiene and, as these are Rh(I) compounds, a counterion, often triflate 21. Both enantiomers of BINAP are available commercially.8... 

Catalytic hydrogenation of the benzylic C-0 bond in 179 removes the second chiral centre and the hydroxy-ester 180 is prepared for coupling to an enantiomerically pure amine by conversion to the triflate 181. The coupling goes with inversion to give enalapril41182. 

Compound 20 was converted to the ido compound 22 in 78% yield. Triflation of 22 followed by displacement with azide ion gave 23. Hydrogenation and subsequent protection with benzyl chloroformate gave the carbamate 24 in 44% overall yield from 22. Removal of the isopropylidene group in 24 followed by catalytic hydrogenation afforded 1. 

Catalytic hydrogenation of 1 in the presence of rhodium on aluminum oxide proceeds smoothly to afford (5)-hexahydromandelic acid (3) [2]. Subsequent treatment of 3 with ethyllithium provides in 75% yield the ketone 4, which is 0-silylated to afford 5. Generated in situ with the appropriate dialkylboron triflate and 5, the boron enolates 6a—c react with a variety of aldehydes to provide exclusively a mixture of syn-d o products 7 and 8 in 70-80% yields, often with excellent stereoselectivities. 

Catalytic hydrogenation of carbonyl compounds to alkanes is a difficult proposition under normal conditions, although limited success is attainable with aromatic ketones. However, certain enolates derived from ketones have been shown to undergo catalytic reduction to alkanes quite efficiently. For example, enol triflates of ketones are reduced over platinum oxide catalyst to alkanes (equation 56) . Similarly, enol phosphates, conveniently prepared from ketones, can be quantitatively hydrogenated to alkanes (equation 57) . ... 

Catalytic hydrogenation of both double bonds yielded the quinolizidine-1,4-dione ( )-2234, the ketone of which was converted into a vinyl triflate before Stille coupling with aUyltributylstannane afforded the conjugated diene ( )-2235. Hydrogenation then produced a 5 2 mixture of racemic di-astereomers 2236 and 2237, which were separated by HPLC. Finally, addition of allylmagnesium chloride to the separated lactams followed by reduction of the adducts with sodium cyanoborohydride gave ( )-quinolizidine 2071 (rac-1728) and ( )-l-ept-quinoHzidine 2071 (rac-2238) as the sole isomers from 2236 and 2237, respectively. 

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