PTOC ester

Bis(tributylstannyl)benzopinacolate PTOC ester or Barton ester... 

The mechanism of aminyl radical generation from PTOC carbamates follows closely the radical chain mechanism of alkyl radical generation from PTOC esters. The chain reaction sequence involves the series of steps shown in Scheme 8. Several methods for inducing N —O bond cleavage are possible. Photochemical decomposition of 29 via visible light irradiation is used to initiate the chain reaction sequence at ambient or subambient temperature reactions have been run as low as -78°C (91TL6493). 

Bergman cyclization 35 PTOC Esters 36 Chlorination 37 Radical clocks 37... 

Recently, it has been demonstrated by Sweeney and co-workers that fi-lactones can also be prepared by radical 4-exo cychsation methodology [93], The intermediate 0-(alkenoyl)oxyalkyl radicals 101 could be prepared by tin hydride treatment of the corresponding (l-bromopropyl)cinnamate. However, best results were obtained with N-hydroxypyridine thione (PTOC) esters 100. In this case a toluene solution of the PTOC ester and BusSnH was slowly added by syringe pump to a heated flask irradiated with a timgsten lamp (Scheme 22). For precursors containing only one substituent at the terminus of the double bond the direct reduction was the main process. For precursors 100 with two phenyl or two arylthio substituents, 4-exo cyclisation of the intermediate C-centred radicals 101 was efficient and good yields of -lactones 103 were isolated, accompanied in each case by minor amounts of the direct reduction product. The bis-arylthio functionahty was particularly appropriate for further FGI. 

White phosphorus in THF reacts in a long radical chain reaction with carbon radicals derived from Barton s PTOC esters. Oxidation of the adducts (HgOg or SOg) provides a convenient synthesis of phosphonic acids. For sensitive products the further transformation to phosphonic acids is best carried out with an excess of SO2. The reaction is illustrated by the synthesis of L-2-amino-4-phosphonobutyric acid in 58% yield from L-glutamic acid using the appropriate protecting groups (Scheme 8.91). ... 

The fundamental steps in the PTOC-thiol method are illustrated in Fig. 4 [10]. The radical source is one of Barton s PTOC esters or a related thiohydroxamic acid derivative [11, 12] these are made from the corresponding carboxylic acids. The acyloxyl radicals produced in the initiation step rapidly decarboxylate to give the radical of interest. This radical reacts with a hydrogen atom donor, a thiol in this... 

Dimethylpyrrolidine and its analogs are efficient stereocontrol elements, since the C2 axis of the pyrrolidine makes the conformation about the carbonyl-nitrogen bond irrelevant. In radicals such as 27, both conformations provide essentially the same stereochemical environment for the radical center. An example of the use of dimethylpyrrolidine, is shown in Fig. 4. In this transformation, a /cr/-butyl ETOC ester is reacted with the acryloyl carboxamide of dimethylpyrrolidine and the addition product is isolated in excellent yield and selectivity. The propagation sequence involves addition of a tert-h xty radical to the acrylamide, trapping of the adduct radical, 27, by the PTOC ester, and decarboxylation of the pivaloyl carboxy radical. This transformation may be initiated thermally or photochemically, and photoinitiation at reduced temperatures gives product with higher diastereoselectivity. A... 

The radical 28 reacts with alkenes, PTOC esters, and allylstannane with good selectivity. The oxazolidine with Ri = t-Bu is particularly effective as an auxiliary group. Thus, the radical 28 having R = (C6Hn)CH2 and Rj = -Bu reacts with allytributylstannane with a selectivity greater the 20 1 at 80 °C, and its reaction with PTOC esters gives a 60 1 mixture of thiopyridyl ester products at room temperature. 

Porter reported the first decarboxylative azidation of thiohydroxamates (Barton PTOC esters). Moderate to good yields of the desired azides were obtained together with the S-pyridyl derivatives resulting from the direct trapping of the radical by the Barton ester. Interestingly, these reactions proved to be diastereoselective as demonstrated by the first example depicted in Scheme 8.12. The formation of the 5-pyridyl derivatives (Barton rearrangement) limits severely the scope of this process. The use of a new type of thio-hydroxamate esters (MPDOC esters) allows to overcome this difficulty (Scheme 8.26). These esters are more stable than the classical Barton esters and less prone to rearrange... 

Structure and Reactivity PTOC Esters Related Thiohydroxamate Systems Preparation of Barton Esters 67-1... 

In the wake of efficient free-radical chemistry by means of PTOC esters, a number of related hydroxamate systems have been devised and tested (Scheme -pj g influence of the substitution pattern of the... 

The use of PTOC esters under photochemical conditions offers a logical solution to the lack of reactivity and to the chain propagation problems, usually encountered in free-radical fragmentation of organob-oranes (Scheme 18). In the postulated mechanism of the reaction, the formed arylcarbonyloxyl radical ... 

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