Biosynthesis of Phenolic Compounds in Grape Berry

The biosynthesis of soluble phenolics begins with the Pentose Phosphate Pathway and/or from Glycolysis and results in the synthesis of the aromatic amino acid phenylalanine, a product of the shikimate pathway.
The first enzyme responsible for the phenolic synthesis is PAL (phenyl ammonia lysase), which converts phenylalanine into cinnamic acid. Cinnamic acid undergoes a series of transformations resulting in the formation of precursors of several simple phenolics, like phenolic acids, lignin precursors, etc.
On a per berry basis, total hydroxycinnamates in mesocarp tissues peak prior to véraison and then decline, leading to a constant amount (per berry) as the fruit ripens. In ripe fruit, the most abundant hydroxycinnamates are caftaric and coutaric acids. The level of hydroxycinnamates in the juice of different vinifera varieties is highly variable, ranging from 16 to 430 mg/L.
The incorporation of 3 molecules of malonyl-CoA, produced via the acetate pathway, with 4-coumaroyl-CoA starts the phenylpropanoid pathway. These precursors generate complex phenolic compounds, like the flavonoids or the stilbenes, depending on the intervening enzyme, chalcone synthase (CHS) for flavonoid synthesis and the stilbene synthase (SS) for stilbene synthesis.
Skin tannins, which are synthesized very early in berry development, change very little on a per berry basis from véraison to harvest, although their concentration declines along berry growth. Qualitative changes, such as the increase of the polymerization degree, can also take place from véraison to harvest.
Generally, there is a decline in seed tannins during ripening that accompanies seed browning, possibly due to tannin oxidation.
Skin anthocyanins, which show little turnover once formed, appear to behave like typical end products. Acquisition of the red/blue colour of red varieties along ripening is a visual indicator of the biochemical processes that occur in grape berries. This reflects more precisely the accumulation of anthocyanin pigments in the vacuoles of skin cells, absent in the white grape varieties.

References:
1. Carlos Conde, Paulo Silva, Natacha Fontes, Alberto C. P. Dias, Rui M. Tavares, Maria J. Sousa, Alice Agasse, Serge Delrot, Hernâni Gerós, Biochemical changes throughout Grape Berry development and fruit and wine quality, Food, 1(1), 1-22 ©2007 Global Science Books.
2. Yung-Fen Huang, Sandrine Vialet, Jean-Luc Guiraud, Laurent Torregrosa, Yves Bertrand,Veronique Cheynier, Patrice This and Nancy Terrier, A negative MYB regulator of proanthocyanidin accumulation, identified through expression quantitative locus mapping in the grape berry, New Phytologist, (2014) 201: 795–809 doi: 10.1111/nph.12557.
The illustration above is a composite from reference 1 (Pentose Phosphate Pathway and Glycolysis) and 2 (General Phenylpropanoid/flavonoid Pathway).