I would like to design 230 V voltage source with frequency about 5 Hz. How to do this, because transformer is not a good idea. The DC source is 6 V. AC voltage source should be about 5W. Any suggestion, maybe link?

 

I don't like your chances ....

One approach might be to first invert / convert the 6V DC to a high voltage DC bus value - this can be done at high frequency and the design is possible.

Once you have the high voltage DC bus, this could be (PWM?) inverted - e.g. direct off a MOSFET H-bridge - to your required 220V at 5Hz. Depends very much on how pure you want the AC waveform. If you need a clean sinusoid then the AC filtering demands will be problematic at 5Hz.

 

One thing in your favor is that you only need a few watts .....