Spiroketal structure

Under certain conditions of propylene/carbon monoxide copolymerisation with certain catalysts, poly(spiroketal) structural units... 

Different mechanistic interpretations of the formation of an alternating propylene/carbon monoxide copolymer of poly(spiroketal) structure were considered [107, 478, 480, 481, 489]. Any reasonable proposal, however, needs to take into account the nature of the end groups in the copolymer chains. To date this has not been possible owing to the low solubility of the copolymer in solvents other than hexafluoroisopropanol however, this solvent, probably because of its acidic nature, causes transformation of the poly(spiroketal) structure into an isomeric poly(ketone) structure [489]. The formation of a cyclic polymeric structure could be favoured by minor entropy loss due to the intramolecularity of the process [480,481] and by the peculiar conformational situation of the poly(ketone) structure [491]. 

Under most conditions, only the simple polypropylene ketone) is formed in propylene/carbon monoxide alternating copolymerisation. Isomerisation of poly(ketone) to poly(spiroketal) can occur, and it may be assisted by cationic palladium species and protonic acids. It must be emphasised that a low reaction temperature favours the formation of a spiroketal structure [107]. At a temperature above the ceiling temperature, the poly(spiroketal) depolymerises to the more flexible and entropically favoured poly (ketone) [481]. 

Now that you know about the anomeric effect, you should add it to your mental array of ways of explaining unexpected results. Here is an example. Many fruit flies have pheromones based around a spiroketal structure, which we could represent without stereochemistry as shown below. You can imagine the spiroketal (that is, an acetal of a ketone made of two rings joined at a single atom) being made from a dihydroxyketone—and, indeed, this is very often how they are made synthetically. But this is a bad representation because these compounds do have stereochemistry, and the stereochemistry is very interesting. 

The relative stability of the two structures seems to be determined by entropic factors. The increase in the size of the olefin substituent [32-35] or of the number of substituents, e.g., cychc olefins such as norbornene [36] or dicyclopentadiene [4], leads to the stabilization of the spiroketal structure, which can survive even in solution. However, a precise determination of the relative stabihty has not been reported. As far as the growing of the copolymer chain is concerned, the mechanistic role, if any, of the spiroketal structure is still not very clear [4, 30]. It is noteworthy that the copolymers are, for the most part, isolated in the spiroketal structure when the copolymerization reaction is regio- and stereoregular. 

Conversely, cephalostatins 5 and 6 (5, 6), in which ring C of the left hand steroid unit is aromatized, exhibited reduced activity in the PS system (10 2-10 3 pgmL 1) suggesting the importance of C - D ring junction and possibly C-22 spiroketal structural integrity to the cytostatic properties of the cephalostatins [19]. 

The relative thermodynamic stability of the two structures is unknown however, it seems to be subject to enthropic factors. The spiroketal structure is particularly stable for norbomenef " or bicyclopentadiene.f" For propene, even if the copolymer is formed with the polyspiroketal structure it transforms easily into the polyketone one when heated or when acidic conditions are applied. The reverse is true for allylbenzene.f For... 

It is remarkable that only the polyketone structure was obtained with (R,S)-BINAPHOS and other related phosphine-phosphite ligands, some of which even contain a basic phosphine moiety (i.e., diethyl or dicyclopentyl),considering that the more basic ligands produce the copolymer in the spiroketal structure. Moreover, the diethyl-substituted phosphine-phosphite system gave a regioirregular copolymer. 

The tricyclic spiroketal structure (68) is found in the antibiotic substances papulacandins and has been synthesised as indicated in Scheme 22 or from the corresponding methyl gluconate... 

Figure 40 Linear and spiroketal structure of propene/carbon monoxide-based polyketones.

Depending on the reaction conditions of PCO and OCO copolymerizations, polyketones are obtained either in a linear or in a spiroketal structure. The equilibrium of the keto- and ketal stmcture depends on the energetic difference between both forms, which is fairly small in OCO copolymers of higher olefins (the stability of the spiroketal form increases with the length of the a-olefin). Investigation of the dynamic behavior was carried out by NMR spectroscopy in solution and in the solid state. The spiroketal form can be converted into the linear poly( 1,4-ketone) at higher temperatures or by treatment with l,l,l,3,3,3-hexafluoro-2-propanol (HFIP) and precipitation (Figure 40). 

Descotes et al. reported an approach to the synthesis of crombenine that involves 8-hydrogen abstraction from an acetal to generate the required spiroketal structure. Unfortunately the required 2,4-dioxy substituents lower the yield from 87% when R = H to 20 and 0% when R = OAc and OMe. Because p-methoxy groups deactivate triplet phenyl alkyl ketones by causing the lowest triplet to become n,n, electronic and conformational effects apparently combine to lower values so much that they barely compete with phosphorescence. Moreover, the bulk of the cyclic ether very likely worsens the conformational problem common to o-alkoxyphenyl ketones. 

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