Books
in black and white
Main menu
Share a book About us Home
Books
Biology Business Chemistry Computers Culture Economics Fiction Games Guide History Management Mathematical Medicine Mental Fitnes Physics Psychology Scince Sport Technics
Ads

Sulfur reagents in organic - Metzner P.

Metzner P., Thuiller A. Sulfur reagents in organic - Academic press, 1994. - 200 p.
Download (direct link): sulfarreagents1994.djvu
Previous << 1 .. 4 5 6 7 8 9 < 10 > 11 12 13 14 15 16 .. 67 >> Next

From [49] with permission.
An interesting transthioacetalization route has been reported [50]. A remarkable feature of the methodology is its selectivity, as the different reactivities of the dithioacetals of aldehydes and ketones make possible a selective deprotection of the latter.
Transthioacetalization with 4-nitrobenzaldehyde
CF3SO,SiMe3 (0.2 mmol) was added to a stirred mixture of the dithioacetal (5) (lmmol) and 4-nitrobenzaldehyde (6) (1-1.5 mmol) in
dichloromethane (5 ml) under nitrogen at room temperature. After completion of the reaction (TLC), saturated aqueous sodium hydrogencarbonate was added. The organic layer was separated, washed with water and dried over sodium sulfate. Removal of the solvent under reduced pressure furnished a residue which was purified either by distillation (kugelrohr) or by chromatography on silica gel using 3% acetone/light petroleum as the eluent to furnish the corresponding carbonyl compound (7) in a fair to very good yield, as in the following examples:
no2
(6)
NOz
(8)
(5)
From [50] with permission.
Anodic oxidation provides also an efficient way for the regeneration of a carbonyl group from its dithioacetal derivative. This method proved to be useful in dethioacetalization of oses [51] and of acyl silanes [52].
Acyl silanes by anodic oxidation of2-alkyl-2-trialkylsilyl-l,3-dithianes
anodic oxidation Î
NaC10./H20-MeCN
4 2 R1 SiMe3
(9) (Þ)
Substrates (9) were obtained by silylation of 2-alky 1-1,3-dithianes.
The general procedure for the electrochemical preparation of (10) is as follows. A solution of (9) (3mmol) in wet acetonitrile (40 ml, 5 vol.% of H20) containing sodium perchlorate (0.25 m) was placed in an undivided electrolysis cell equipped with a platinum plate anode and a platinum plate cathode. The system was subjected to a constant current electrolysis (300 mA; current density, ~20mAcm-2) at ambient temperature. After 4 faradays per mole of (9) had been consumed, the electrolysed solution was poured into water (50 ml) and extracted with dichloromethane (3X30ml). The organic layer was dried with magnesium sulfate and concentrated under reduced pressure. The residue was chromatographed on silica gel to afford (10) in an excellent yield.
Examples are Me3SiCO(CH5)2Ph and Me3SiCOCH—CH(CH2)5CH3 ((?) -isomer), obtained in 95 and 84% isolated yields, respectively.
From [52] with permission.
A photosensitized cleavage of dithianes and dithiolanes by visible light [53,54] deserves to be mentioned.
Photolytic transformation of dithioacetals into carbonyl compounds
The usual procedure involves irradiation of a 1:1 MeCN/H,0 solution of dithioacetal (3.0g, —15 mmol), methylene green (182mg. 0.50mmol) and magnesium perchlorate (1.1 g, 5.0 mmol) in a 500 ml round-bottomed flask equipped with a reflux condenser. The solution was purged with nitrogen throughout the reaction (~3-4 h), which was induced by irradiation with a GE Miser 120 W spotlight. The products were isolated by chloroform extraction and flash chromatography in excellent yields (routinely in excess of 90%). Example: R1 = Ph, R2 = Me, n = 0, yield 91%.
From [53] with permission.
The overall cleavage is of a hydrolytic nature. See [53] for a discussion of the mechanism.
The synthesis of tailor-made homochiral dithioacetals has been mentioned previously (see [11] for De Lucchi’s dithiepine (3) in Section 2.1).
O,Õ-Acetals and their S-oxides have also received much attention for their synthetic potentialities [55].
A number of syntheses have been proposed, mainly from a-chlorosulfides and alcohols or from 0,0-acetals or 5,5-acetals. Many references on these methods are to be found in a paper dealing with a direct synthesis of 0,5-acetals from aldehydes [56].
n=0,l
OR2
Î TMSOTf as cat. 1
H,A„ * lfe»SlOR2 * Me>S1SPh ia-h"
37-93%
Eliel’s oxathiane reagent, derived from (fl)-pulegone, is a valuable template for asymmetric syntheses. A large-scale preparation has appeared in Organic Syntheses [57].
- Q4 -
Î
yO-(+)-pulegone (+)-hexahydro-4,4,7-trimethyl-4//-l,3-benzoxathim
A 1,3-oxathiane chiral auxiliary derived from camphor has also been reported [58].
2.5 KETENEDITHIOACETALS AND a-OXOKETENEDITHIOACETALS
r2 SR4 SR4
(1) (2)
Ketenedithioacetals (1) can formally be considered as 5,5-dialkylated dithioacids. However, their reactivity pattern is quite different from that displayed by compounds like dithioacids and dithioesters bearing a thiocarbonyl group, and their syntheses and properties are usually treated apart. The review by Kolb [59] and its 254 references give access to most of the literature in the field (see also [60]). a-Oxoketenedithioacetals (2) are particularly versatile compounds as three-carbon synthons in organic synthesis, and some reviews deal specifically with them [61, 62]. Many efficient procedures are available for their syntheses [59, 61], and number of these make use of the reactions of carbon disulfide with carbanionic species followed by alkylation at the sulfur atoms (see [63] and [64] for the reactions of CS2).
Previous << 1 .. 4 5 6 7 8 9 < 10 > 11 12 13 14 15 16 .. 67 >> Next