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Sulfur reagents in organic - Metzner P.

Metzner P., Thuiller A. Sulfur reagents in organic - Academic press, 1994. - 200 p.
Download (direct link): sulfarreagents1994.djvu
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From [267] with permission.
Organosulfur Compounds as Intermediates in Organic Synthesis
4.1 A WORD OIM DESULFURIZATION
When sulfur chemistry is used as an auxiliary tool in synthesis, the removal of sulfur is a necessary and important step for the whole process. The search of general or selective methods of desulfurization led to the development of many procedures and reagents. A comprehensive review of the subject including applications has been published by Belenkii [268]. In spite of their importance we shall not deal separately with those desulfurization techniques, many examples of which will be encountered in the experimental procedures described throughout this book. We would like, however, to underline the fundamental role played by nickel and its derivatives in reductive desulfurization. Raney nickel is the most classic and widely used reagent for the reductive cleavage of carbon-sulfur bonds, and preparations for various activities are available [269]. Nickel boride and the more recently developed nickel-containing complex reducing agents (see [270] for NICRA and NICRAL, [271] for nickelocene-lithium aluminium hydride) offer alternatives to Raney nickel and its many drawbacks.
4.2 C-C BOND-FORMING REACTIONS 4.2.1 Use of sulfides, dithioacetals and orthotrithioesters
4.2.1.1 1,3-Dithianes as acyl anion equivalents
The use in organic synthesis of carbanions stabilized by one or more adjacent thioether groups is now a well-established technique [272-275]. A landmark in this development was the work of Corey and Seebach on 1,3-dithianes [276]. They pointed out the use of the anions of 1,3-dithianes as
acyl anion equivalents (Umpolung, see Section 2.4). As shown helow, starting from an aldehyde, the formation of a dithiane allows metallation and reaction with an electrophile. Unmasking the carbonyl group leads to the compound which corresponds formally to an electrophilic attack on the carbon of the carbonyl function.
A considerable number of electrophiles were used, and the dithiane route found great utility for the syntheses of simple monofunctional compounds as well as for polyfunctional molecules, for which the dithiane moiety affords an invaluable temporary protection of a future carbonyl group. Some experimental procedures published in Organic Syntheses cyclobutanone [277] and 3-hydroxy-l-cyclohexene-l-carboxaldehyde [278] are illustrative. A similar route to aldehydes [279] makes use of sym-trithiane as a formyl anion equivalent.
>
E
"hydrolysis"
H
) n-BuLi

dithiane. n-Buli j-[(
Q
HgO. BF3:OEt2
CHO
THF
OH
HgCl2, H20
MeO H3o
Vn-C14H29
MeO
n-C14H29-CHO
n-pentadecanal
MeOH
In the following example the ring opening of TV-tosyl aziridines was used to prepare homochiral p-tosylaminocarbonyl compounds [280] in good to excellent yields.
1 X
r\ THF, -78 to 0C / Mel, acetone \
pN_Ts ------------------------^ I -----reflux
j' ii)H20 NHTs r1 NH1
R'= CH2Ph R2= H, Me
CH2CHMe2 CH2Ph. SiMe3
COjH
Ring opening of aziridine with dithianes
To a solution of dithiane in THF, under N2, at -23C was added n-BuLi (1 eq) and the solution was stirred at -23C for 1.3h. The colourless solution was then cooled to -78C and a solution of aziridine (0.9 eq in THF) added (in some cases, instense colours are formed at this stage). The solution was stirred for lh at -78C and then at 0C until all the starting materials had been consumed, as shown by TLC (typically 2h reaction time). The reaction was then quenched at 0C by the addition of H20, and the resulting solution was extracted with EtOAc. The combined organic layers were washed with brine, dried (Na2S04), filtered and the solvent removed in vacuo to yield the crude product. Purification by silica gel chromatography gave analytically pure 2-tosylaminodithianes in yields from 59 to 92% (10 examples).
From [280] with permission.
Hydrolytic removal of dithiane
To a solution of 2-tosylaminodithiane in acetone was added methyl iodide (typically a 10-fold excess) and the solution was heated under reflux for 1 h. A few drops of water were then added and heating was continued for 4h. The resulting mixture was partitioned between EtOAc and H20. The organic layer was washed with brine, dried and the solvent was removed in vacuo to yield crude 2-tosylaminocarbonyl compounds. Chromatographic purification of these compounds gave analytically pure
2-tosylaminocarbonyl compounds in yields from 42 to 100% (eight examples).
From [280] with permission.
We have discussed previously (see Section 2.4) some aspects of the dithioacetalization step involved in this strategy. The presence of the sulfur atoms offers another possibility as a methylene unit results from desulfurization by Raney nickel, dissolved metal or LiAlH4/ZnCl2 reduction [281].
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