Seco (tetrapyrrole)
Prefix in organic chemistry nomenclature
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The prefix seco in tetrapyrrole nomenclature[1] signifies that the macrocyclic ring of a porphyrin, chlorophyll, or corrin is opened at the specified position to the corresponding acyclic (linear) tetrapyrrole.[2] This nomenclature reflects chemical reactions involving respective ring openings and ring closures.
Seco-tetrapyrroles as products of biological and chemical degradation
Linear tetrapyrroles originating from natural chlorophyll breakdown during plant senescence, known only since the 1990s, were first named as 4,5-seco-phytoporphyrins, since about 2014 as phyllobilins, with this systematic name expressing their origin from chlorophyll and their structural relation to bile pigments (fig. 1).[3][4]
Chemical ring cleavage of cyclic tetrapyrroles also produces seco-derivatives, as in the photooxygenolysis of vitamin B12 (fig. 2).[5][6]
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Seco-corrins in corrin syntheses
In the Harvard (Woodward)/ETH (Eschenmoser) variant of the two total syntheses of vitamin B12,[7][8] the corrin macrocyle is built by coupling an A/D- (Harvard) and a B/C (ETH) component, first at meso-C-15 between rings D and C, followed by ring closure of the resulting seco-corrin at C-5, using the Eschenmoser sulfide contraction developed at ETH for both coupling reactions (fig. 3).[9][10]
Figure 3. Corrins from seco-corrins via A/B ring closure by Eschenmoser sulfide contraction:[9] important step in the Harvard/ETH variant of the total syntheses of vitamin B12
Figure 4. Corrin from A/D-seco-corrin via photochemical A/D ring closure:[11][12] key step in the ETH variant of the total syntheses of vitamin B12
In the photochemical ETH variant of these total syntheses, the corrin ring is closed directly between rings A and D – as typical for corrins, meso position C20 missing – in an A/D-seco-corrin → corrin cycloisomerization (fig. 4).[13][12]
Other corrins were synthesized by Eschenmoser et al. from seco-corrins by A/B ring closure via imidoester–enamine C–C condensation,[14] sulfide contraction,[10] or by photochemical A/D cyclization.[15]