Methylated selenium derivatives, methylation

The preparation of nine 75Se derivatives of steroids has been described these labelled steroids have proved useful in assaying the comparative binding of proteins.77 Reaction of 75Se02 with dehydroepiandrosterone yields a selenium derivative (183) which upon reduction with dithiothreitol or HSCH2CH2OH and then methylation furnishes the corresponding methylseleno-steroid. 

Methylseleninic acid (=C H 3-Se(=0)-0 H) (selenium derivative) Methylselenol (— Methyl-SeH) (senenol) Mimosine ( Leucaenol) (pyridinone amino acid)... 

Selenium dioxide oxidation of methyl derivatives (Scheme 30). 

Intramolecular chalcogen interactions may also stabilize reactive functional groups enabling the isolation of otherwise unstable species or their use as transient intermediates, especially in the case of selenium and tellurium. For example, tellurium(II) compounds of the type ArTeCl are unstable with respect to disproportionation in the absence of such interactions. The diazene derivative 15.23 is stabilized by a Te N interaction. Presumably, intramolecular coordination hinders the disproportionation process. Other derivatives of the type RTeX that are stabilized by a Te N interaction include 8-(dimethylamino)-l-(naphthyl)tellurium bromide, 2-(bromotelluro)-A-(p-tolyl)benzylamine, and 2-[(dimethylammo)methyl]phenyltellunum iodide. Intramolecular donation from a nitrogen donor can also be used to stabilize the Se-I functionality in related compounds." ... 

There have been no reports of complexes of " JV-substituted thiosemicarbazones derived from 2-formylpyridine, but 2-acetylpyridine JV-methyl-thiosemicarbazone, 3a, formed [Fe(3a-H)2]C104 and [Fe(3a-H)2]FeCl4 [117]. The nature of these two species was established by partial elemental analyses, molar conductivities, magnetic moments, electronic, infrared, mass and electron spin resonance spectra. A crystal structure of a related selenosemicarbazone complex confirmed the presence of a distorted octahedral iron(III) cation coordinated by two deprotonated anions so that each ligand is essentially planar and the azomethine nitrogens are trans to each other the pyridyl nitrogen and selenium donors are both cis. 

Simple derivatives of the parent compound in this group, exo,endo- and o ,o -2,6-dimethyl-l-phosphabicyclo[2.2.1]-heptane 211 and 212, have been synthesized by the cyclization of 2-methyl-4-(2-propenyl)phospholane in the presence of base <2002ZFA580>. The structures were determined via spectroscopic means as well as X-ray crystallography and further confirmed by reactions with selenium, sulfur, (CH3)2SeO, CH3I, and HS03F. 

Selenolo[3,2-h]selenophene (12) and selenolo[2,3-b]selenophene (13) have been synthesized from lithium derivatives of 2-(3-bromo-2-selienienyl) 1,3-dioxane and 2-(3-selenienyl) 1,3-dioxane, respectively, by reaction with selenium and methyl chloroacetate followed by Dieckmann cyclization.46 Even the third classical selenophthene (11) has been synthesized by two different routes, using 2,3-bischloromethyl-5-carbomethoxyselenophene (14) or preferably 4-methylseleno-3-selenophene aldehyde (IS).46 The fourth selenophthene isomer (16), which has a nonclassical structure, has not yet... 

An alternative practical synthesis of triquinacene-2-carboxylic acid (as the dextrorotatory enantiomer) has l n described by Deslongchamps and Soucy Their protocol begins with hydroxy ketone 467 and passes via the 2-methyl derivative (Scheme XXXVIII). Selenium dioxide oxidation of the hydrocarbon provided the aldehyde which was further oxidized and then hydrolyzed to arrive at the add. 

Benzo[ ]perimidinone carbaldehyde and carboxylic acid derivatives have also been prepared by alkyl oxidation. Thus, treatment of the 2-methyl derivative 588 with selenium dioxide at reflux in dioxane gave the aldehyde 589, which then gave carboxylic acid 590 on reaction with sodium chlorite <2001JME2004>. 

The Se-(4-methoxybenzyl)selenocysteine is obtained by reduction of selenocystine with NaBH4 and in situ reaction with 4-methoxybenzyl chloride. 7 The optimized procedure of Tanaka and Soda 32 is preferentially used for the synthesis of the starting selenocystine, which involves reaction of (1-chloroalanine with a 2.3-fold excess of disodium diselenide in aqueous solution at pH 9. Alternatively, the significantly less selenium demanding synthesis of Stocking et al. 33 is used for the preparation of expensive 77Se-selenocystine, this consists of the reaction of methyl (2R)-2-[(/ert-butoxycarbonyl)amino]-3-iodopropanoate with equivalent amounts of dilithium diselenide. Subsequent conversion of SeC(Mob) into the M -Fmoc derivative 7 and finally into the pentafluorophenyl ester 10 is performed following standard procedures. 

A chemical correlation of the (—)-methyl derivative 109 with the ( )-methylester 111 was possible by selenium dioxide oxidation of the former and subsequent silver... 

Methyl groups in the 2-, 4- or 6-position of pyrimidine are also more reactive. In addition to typical reactions such as condensation with benzaldehyde, selenium dioxide oxidation and halogenation, they can be converted into oximino groups by nitrous acid, and undergo Claisen condensation with (C02Et)2. In the reaction of 2,5-dimethylpyrimidine with benzaldehyde, only the electrophilic 2-methyl group reacts preferentially to yield the 2-styryl derivative (631). In quinazolines partial double bond fixation makes a methyl group in the 4-position more reactive than that in the 2-position. 

Oxidation of methyl ketone guanylhydrazones (686) with selenium dioxide affords the corresponding monosubstituted glyoxal derivatives (687) which cyclize to the 5-unsubstituted 3-amino-1,2,4-triazines (473) (78HC(33)189, p.360). We have already noted the cyclization of bis(alkylidene)- or bis(arylidene)-acetone guanylhydrazones (475) in the synthesis of 6-(vinyl-substituted) 3-amino-1,2,4-triazines (477) (Section 2.19.4.1.1). 

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