Lophocerine

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Lophocerine is an isoquinoline alkaloid reported from the cactus Lophocereus schottii. It belongs to the group of substituted tetrahydroisoquinoline alkaloids and has been investigated in research on cactus chemistry and alkaloid biosynthesis. The compound was first reported in a 1958 study by Carl Djerassi, Takashi Nakano and James M. Bobbitt, which described its isolation and proposed chemical structure. Subsequent investigations examined its chemical synthesis and the biological processes involved in its formation in the plant.

Chemical structure and classification

Lophocerine is classified as an isoquinoline alkaloid, a broad class of nitrogen-containing natural products whose structures contain an isoquinoline-derived ring system. More specifically, lophocerine has a tetrahydroisoquinoline skeleton. Its reported structure contains an isobutyl substituent, an N-methyl group, a methoxy group and a hydroxy group.

The structure proposed in the original isolation study was 1-isobutyl-2-methyl-6-methoxy-7-hydroxy-1,2,3,4-tetrahydroisoquinoline. The researchers obtained lophocerine in an impure form from the phenolic alkaloid fraction of the plant. They supported the proposed structure by methylating the compound and obtaining a derivative identified as a known dimethoxy tetrahydroisoquinoline.

The compound is recorded in PubChem, the chemical database maintained by the United States National Center for Biotechnology Information. The record associates lophocerine with PubChem Compound Identifier (CID) 442313 and molecular formula C₁₅H₂₃NO₂. The formula corresponds to a molecular mass of approximately 249.35 g/mol.

Discovery and isolation

Lophocerine was reported during a systematic investigation of alkaloids from Lophocereus schottii published in 1958. In the same study, Djerassi, Nakano and Bobbitt described piloceredine, another cactus alkaloid, in addition to the previously known pilocereine. Their work formed part of the broader chemical investigation of the cactus and its naturally occurring constituents.

The researchers isolated lophocerine from the plant's phenolic alkaloid fraction. The material was not obtained in a completely pure form, so the proposed structure relied on chemical evidence and the behaviour of derivatives prepared from the isolated material. In particular, methylation produced a dimethoxy derivative that was consistent with the assigned substitution pattern on the tetrahydroisoquinoline ring.

Chemical synthesis

In 1959, James M. Bobbitt and Tsu-Teh Chou published a paper describing the synthesis of lophocerine as part of a series of investigations into isoquinoline alkaloids. The publication provided a synthetic route to the compound and complemented the earlier work in which lophocerine had been isolated from cactus material.

Biosynthesis

Research published during the late 1960s and early 1970s investigated how lophocerine is formed in Lophocereus schottii.

In a 1968 paper, D. G. O'Donovan and H. Horan studied lophocerine biosynthesis in intact plants. Their results indicated that tyrosine contributes to the C₆–C₂–N portion of the tetrahydroisoquinoline framework, while leucine and mevalonic acid contribute to the remaining five-carbon unit. The findings were reported as consistent with the proposed biogenetic pathway for the compound.

A subsequent paper by O'Donovan and Horan in 1969 examined the biosynthetic relationship between lophocerine and pilocereine. The researchers observed the conversion of labelled lophocerine into pilocereine in Lophocereus schottii through oxidative phenolic coupling. Their experiments also implicated L-methionine as the source of the N- and O-methyl groups of lophocerine.

In 1974, O'Donovan and Edward Barry published a further study of lophocerine biosynthesis in intact plants. They reported that the five-carbon unit of the alkaloid probably arises independently from leucine and mevalonic acid. Their investigation also identified 3-methylbut-3-enyl pyrophosphate, 3-methylbutan-1-ol and 3-methylbutanal as intermediates in the pathway under study.

Occurrence and phytochemical research

Published research has identified lophocerine in chemical investigations of Lophocereus schottii.

A 2022 study by Arturo Orozco-Barocio and colleagues examined ethanolic extracts and fractions of Lophocereus schottii using ultra-performance liquid chromatography–mass spectrometry. The researchers investigated the chemical composition of the preparations and their activity in laboratory experiments involving cancer cell models.

Relationship to other cactus alkaloids

Lophocerine has been studied alongside pilocereine and piloceredine, compounds reported from Lophocereus schottii. The original isolation paper discussed a possible biosynthetic relationship between lophocerine and the other alkaloids, while subsequent experiments examined the conversion of lophocerine into pilocereine.

References