Chlorins, oxidative ring opening

Oxidative ring-opening with pyridinium chlorochromate (PCC) gives p, y-unsaturated ketones (equation 24). Formation of chlorinated byproducts could be suppressed by employing certain additives (AgNOj, H20). However, bicyclic cyclopropanes afford only moderate yields. 

Oxidative cleavage of the l,2-bis(siloxy)cyclopropane 8 with aqueous sodium periodate similarly affords 1,3-dione 9, in this case in 37% yield.Upon treatment with iron(III) chloride, 1 -(methylsulfanyl)cyclopropanol undergoes a similar chlorination with ring opening to give a / -chlorinated thioester. ... 

The isolation of chlorin Pe and mesochlorin Pe and possibly the purpurin 18 intermediates suggests that allomerization is an important process for chlorin formation by oxidative ring opening (Figure 1.7). It is also possible that petroporphyrins of the etio and phyllo type can be derived from these chlorins. 

True electrophilic substitution is very difficult in pyridopyridazines. For example, the [3,4-d] parent (286) is inert to hot 65% oleum (68AJC1291), and although formation of a 3-bromo derivative (308) was reported in the [2,3-d] series, it seems to have arisen by an addition-elimination reaction via the dibromide (309) (69AJC1745). Attempted chlorination led to ring opening. A similar effect was observed in the [3,4-d] system, where an 8-bromo derivative was obtained (77BSF665), and in iV-oxides of the pyrido[2,3-c]pyridazine and fused pyridazino[3,4-c]isoquinoline series (72JHC351). The formation of (311) from (310)... 

Next to the above presented use of SiCl for the in situ preparation of a Lewis acid catalyst with a Lewis base for the aldol reaction, it is possible to apply this compound as a reagent in the ring opening of epoxides leading to chlorinated alcohols. Denmark [104] reported that the chiral phosphoramide 38 catalyzed the asymmetric ring opening reaction of meso-epoxides in the presence of tetrachlo-rosilane. Similar examples were provided by Hashimoto in 2002 [105], applying the A -oxide 39 as catalyst (Scheme 30). 

Electrophilic chlorination of 1,3,2-dioxathiolane 2-oxides gives rise to ring-opened products for example, chlorination of 1,3,2-dioxathiolane 2-oxide afforded 2-chloroethyl chlorosulfate <67JCS(C)314>. 

The mechanism of the reaction of thiophene with a variety of radicals as a function of pH has been studied using ESR (81JCS(P2)207). Attack by -OH at pH 6 proceeds by direct addition with a preference to add to the a-position the ratio of (226) to (227) is 4 1. At low pH the (3-adduct easily loses OH- to form the thiophene radical-cation, which may undergo rehydration. In the case of 2-methyIthiophene the radical-cation deprotonates to give the thenyl radical this is reminiscent of the electrochemical oxidation (Section 3.14.2.6). The radical-cations are also formed by direct electron abstraction from the thiophene substrates by chlorine anion-radicals. At pH >6, (226) starts disappearing with formation of ring-opened products (Scheme 61). 

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